Determining moving properties of a target in an extreme ultraviolet light source

ABSTRACT

A moving property of a target is measured as the target travels toward a target space. The target including a component that emits light when converted to a plasma. A diagnostic probe system is interacted with a current target moving toward the target space. The interaction occurs prior to the current target entering the target space and after an immediately preceding target has interacted with a prior radiation pulse in the target space. First and second light that is produced at least in part from the interaction between the diagnostic probe system and the current target is detected prior to the current target entering the target space and after an immediately preceding target has interacted with the prior radiation pulse in the target space. One or more moving properties of the current target are determined based on an analysis of the detected first and second light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/555,888, filed on Aug. 29, 2019, entitled Determining MovingProperties of a Target in an Extreme Ultraviolet Light Source; which isa continuation of U.S. application Ser. No. 16/322,002, filed Jan. 30,2019, entitled Determining Moving Properties of a Target in an ExtremeUltraviolet Light Source, and issued as U.S. Pat. No. 10,429,173 on Oct.1, 2019; which is a national phase of PCT/US2017/050559, filed on Sep.7, 2017, entitled Determining Moving Properties of a Target in anExtreme Ultraviolet Light Source; which claimed priority to U.S.application Ser. No. 15/265,376, filed on Sep. 14, 2016, entitledDetermining Moving Properties of a Target in an Extreme UltravioletLight Source, and issued as U.S. Pat. No. 9,778,022 on Oct. 3, 2017. Allof these applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The disclosed subject matter relates to a system and method formeasuring aspects of a target along its trajectory in a laser producedplasma extreme ultraviolet light source.

BACKGROUND

Extreme ultraviolet (EUV) light, for example, electromagnetic radiationhaving wavelengths of around 50 nm or less (also sometimes referred toas soft x-rays), and including light at a wavelength of about 13 nm, canbe used in photolithography processes to produce extremely smallfeatures in substrates, for example, silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material that has an element, for example, xenon,lithium, or tin, with an emission line in the EUV range in a plasmastate. In one such method, often termed laser produced plasma (“LPP”),the required plasma can be produced by irradiating a target material,for example, in the form of a droplet, plate, tape, stream, or clusterof material, with an amplified light beam that can be referred to as adrive laser. For this process, the plasma is typically produced in asealed vessel, for example, a vacuum chamber, and monitored usingvarious types of metrology equipment.

SUMMARY

In some general aspects, a method is performed for measuring a movingproperty of a current target as it travels along a trajectory toward atarget space. The method includes: detecting a plurality oftwo-dimensional representations of light that are produced due to aninteraction between the current target and each of a plurality ofdiagnostic probes prior to the current target entering the target space;determining one or more moving properties of the current target based onan analysis of the detected plurality of two-dimensional representationsof light, the determining being completed prior to the current targetentering the target space; and, if the determined one or more movingproperties of the current target are outside an acceptable range,adjusting one or more characteristics of a radiation pulse directed tothe target space.

Implementations can include one or more of the following features. Forexample, the method can also include interacting the radiation pulsewith a present target in the target space, wherein the present target iseither the current target that has entered the target space or anothertarget that has entered the target space, and wherein the other targetenters the target space at a time that follows the time when the currenttarget enters the target space. The other target can be adjacent to thecurrent target along the trajectory. The other target can be adjacent toan intermediate target that is between the other target and the currenttarget along the trajectory.

The radiation pulse can convert at least part of the present target intoplasma that emits extreme ultraviolet light when the radiation pulseinteracts with the present target. The radiation pulse can deliverenergy to the present target to modify a geometric distribution of thepresent target.

The method can include releasing the current target along the trajectorytoward the target space, which is defined within a laser-produced plasmaextreme ultraviolet light source.

The two-dimensional representation of the light that is detected can bea two-dimensional image of the light.

The method can include directing at least one diagnostic probe towardthe current target along a plane defined by a first directionperpendicular to an axial direction and the axial direction, wherein thecurrent target travels along a direction that has a component along theaxial direction. The radiation pulse can be directed toward the targetspace along a second direction perpendicular to the axial direction andto the first direction.

The method can also include directing the diagnostic probes toward tothe current target so that each diagnostic probe interacts with thecurrent target at a distinct diagnostic location prior to the currenttarget entering the target space.

Each diagnostic probe can be a diagnostic light beam. The light that isproduced due to the interaction between the current target and eachdiagnostic probe can include the diagnostic light beam scattering off asurface of the current target. The light that is produced due to theinteraction between the current target and the diagnostic probe caninclude a shadow of the current target obscuring at least a portion ofthe diagnostic light beam.

The one or more moving properties of the current target can bedetermined by determining one or more of a position, a velocity, and anacceleration of the current target. The one or more moving properties ofthe current target can be determined by determining one or more movingproperties of the current target along each dimension of a threedimensional coordinate system. The one or more characteristics of theradiation pulse can be adjusted by adjusting one or more of a timing ofa release of a radiation pulse and a direction at which the radiationpulse travels.

The method can include detecting a plurality of one-dimensional valuesof the light that are produced due to the interaction between thecurrent target and each diagnostic probe prior to the current targetentering the target space.

The current target can interact with the plurality of diagnostic probesafter a prior and adjacent target has interacted with a prior radiationpulse in the target space. The current target can interact with theplurality of diagnostic probes while the current target is beinginfluenced at least in part by plasma pushback forces.

The method can include analyzing the detected plurality oftwo-dimensional representations of light, the analyzing including:identifying one or more regions of interest within each representation,each region of interest corresponding to a location of the currenttarget along the trajectory; for each region of interest, determining acentral area of the region of interest; and deriving a position of thecurrent target in three dimensions based on the determined centralareas.

In other general aspects, a method is performed for measuring one ormore moving properties of each target in a plurality of targets as eachtarget travels along its trajectory toward a target space. The methodincludes: prior to each target of the plurality reaching the targetspace, and after a prior and adjacent target has entered the targetspace, interacting a plurality of diagnostic probes with each target ofthe plurality of targets at diagnostic locations along that target'strajectory. For each target in the plurality of targets: a plurality oftwo-dimensional representations of light produced due to theinteractions between that target and the diagnostic probes are detected;the detected two-dimensional representations are analyzed; one or moremoving properties of that target are determined along each dimension ofa three dimensional coordinate system based on the analysis of thedetected two-dimensional representations; and it is determined whetherone or more characteristics of a radiation pulse directed to the targetspace need to be adjusted based on the determined one or more movingproperties.

Implementations can include one or more of the following features. Forexample, the method can include, for each target in the plurality oftargets: detecting a time associated with each interaction between thattarget and a diagnostic probe; analyzing the detected times; anddetermining one or more moving properties of the target along at leastone of the dimensions of the three dimensional coordinate system basedon the analysis of the detected times.

The one or more moving properties of a target can be determined bydetermining one or more of a position, a velocity, and an accelerationof the target.

The two-dimensional representation of the light can be detected bydetecting a two-dimensional image of the light. The detectedtwo-dimensional image can be analyzed by identifying one or more regionsof interest within the image and calculating a centroid for eachidentified region of interest.

Determining whether one or more characteristics of the radiation pulseneed to be adjusted can include determining that one or morecharacteristics of the radiation pulse need to be adjusted if thedetermined one or more moving properties of the target are outside anacceptable range. The one or more characteristics of the radiation pulsecan be adjusted by adjusting one or more of a timing of a release of theradiation pulse and a direction at which the radiation pulse travels.

The two-dimensional representations of light can be detected prior tothe target entering the target space. The detected two-dimensionalrepresentations can be analyzed prior to the target entering the targetspace. And, the one or more moving properties of the target can bedetermined prior to the target entering the target space.

The target can interact with the plurality of diagnostic probes whilethe target is being influenced at least in part by plasma pushbackforces.

In other general aspects, an apparatus includes: a target deliverysystem configured to release a target toward a target space, the targetincluding a material that emits extreme ultraviolet (EUV) light whenconverted to plasma; a chamber that defines the target space and aregion between the target delivery system and the target space, thetarget space positioned to receive a plurality of radiation pulses, eachradiation pulse that interacts with a target in the target space causingat least part of that target to be converted into plasma that emits EUVlight; a diagnostic system; and a control system. The diagnostic systemincludes: a probe module that produces a plurality of diagnostic probes,each diagnostic probe interacting with the target in the region prior tothe target entering the target space; and a detection module thatdetects a plurality of two-dimensional representations of light that isproduced from the interaction between the diagnostic probes and thetarget. The control system is connected to the diagnostic system and isconfigured to: receive the plurality of two-dimensional representationsfrom the detection module; analyze the received two-dimensionalrepresentations; and determine one or more moving properties of thetarget based on the analysis.

Implementations can include one or more of the following features. Forexample, the control system can be configured to adjust one or morecharacteristics of a radiation pulse directed to the target space if thedetermined one or more moving properties of the target are outside anacceptable range. Each diagnostic probe can be a diagnostic light beam.The light produced due to the interaction between the target and eachdiagnostic probe can include the diagnostic light beam scattering off asurface of the target. The light produced due to the interaction betweenthe target and each diagnostic probe can include a shadow of the targetobscuring at least a portion of the diagnostic light beam.

DRAWING DESCRIPTION

FIG. 1A is a block diagram of a laser produced plasma extremeultraviolet light source including a diagnostic system for detecting amoving property of a target traveling in an extended target regiontoward a target space along the −X direction;

FIG. 1B is a schematic diagram showing a view of the light source ofFIG. 1A in which the X direction is coming out of the page and thetarget trajectory is into the page;

FIG. 2A is a schematic diagram showing a point in time just before aprior radiation pulse and a prior target interact with each other at atarget location within a target space of the EUV light source of FIG. 1;

FIG. 2B is a schematic diagram showing a point in time just before acurrent radiation pulse and a current target interact with each other atthe target location within the target space of the EUV light source ofFIG. 1;

FIG. 3 is a block diagram of an exemplary diagnostic system of the EUVlight source of FIG. 1;

FIG. 4 is a block diagram of an exemplary control system of the EUVlight source of FIG. 1;

FIG. 5 is a block diagram of an exemplary diagnostic system of the EUVlight source of FIG. 1;

FIG. 6 is a block diagram of an exemplary control system of the EUVlight source of FIG. 1;

FIG. 7 is a block diagram of an exemplary diagnostic system of the EUVlight source of FIG. 1;

FIG. 8 is a block diagram of an exemplary control system of the EUVlight source of FIG. 1;

FIG. 9A is a schematic diagram showing a close up of the interactionbetween a diagnostic radiation beam and a current target in which thediagnostic radiation beam axis is generally perpendicular to atrajectory of the current target and the current target trajectory isaligned with an X direction;

FIG. 9B is a schematic diagram showing a close up of the interactionbetween a diagnostic radiation beam and a current target in which thediagnostic radiation beam axis is generally perpendicular to atrajectory of the current target and the current target trajectory isoffset from an X direction along a Y direction;

FIG. 9C is a schematic diagram showing a close up of the interactionbetween a diagnostic radiation beam and a current target in which thediagnostic radiation beam is directed along an axis that is in the XYplane and the current target trajectory is aligned with the X direction;

FIG. 9D is a schematic diagram showing a close up of the interactionbetween a diagnostic radiation beam and a current target in which thediagnostic radiation beam is directed along an axis that is in the XYplane and the current target trajectory is offset from the X directionalong the Y direction;

FIG. 10 is a block diagram of an exemplary diagnostic system of the EUVlight source of FIG. 1;

FIG. 11 is a schematic diagram showing a preliminary radiation pulsedirected to a first target location and a main radiation pulse directedto a second target location for interaction with the current target ofthe EUV light source of FIG. 1;

FIG. 12 is a block diagram of an exemplary optical source for use in theEUV light source of FIG. 1;

FIG. 13 is a flow chart of an exemplary procedure performed by the EUVlight source (under control of the control system) for determining amoving property of a current target in the extended target region;

FIG. 14A is a schematic diagram of an exemplary diagnostic system,extended target region, and target space as viewed along the Z directionof the EUV light source of FIG. 1, showing a point in time just before aprior radiation pulse and a prior target interact with each other at atarget location within a target space;

FIG. 14B is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 14A as viewed along theX direction, showing the same point in time as FIG. 14A;

FIG. 15A is a schematic diagram of the exemplary diagnostic system,extended target region, and target space as viewed along the Z directionof the EUV light source of FIG. 1, showing a point in time just afterthe prior radiation pulse and the prior target interact with each otherat the target location within the target space;

FIG. 15B is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 15A as viewed along theX direction, showing the same point in time as FIG. 15A;

FIG. 16A is a schematic diagram of the exemplary diagnostic system,extended target region, and target space as viewed along the Z directionof the EUV light source of FIG. 1, showing a point in time when thecurrent target interacts with a first diagnostic light beam of thediagnostic system within the extended target region;

FIG. 16B is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 16A as viewed along theX direction, showing the same point in time as FIG. 16A;

FIG. 17A is a schematic diagram of the exemplary diagnostic system,extended target region, and target space as viewed along the Z directionof the EUV light source of FIG. 1, showing a point in time when thecurrent target interacts with a second diagnostic light beam of thediagnostic system within the extended target region;

FIG. 17B is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 17A as viewed along theX direction, showing the same point in time as FIG. 17A;

FIG. 18A is a schematic diagram of the exemplary diagnostic system,extended target region, and target space as viewed along the Z directionof the EUV light source of FIG. 1, showing a point in time after thecurrent target has interacted with the second diagnostic light beam inthe extended target region and during which the current radiation pulseis being directed to the target space;

FIG. 18B is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 18A as viewed along theX direction, showing the same point in time as FIG. 18A;

FIG. 19A is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of the EUV light source of FIG.1 as viewed along the Z direction, showing a point in time during whichthe current target is interacting with the current radiation pulse inthe target space;

FIG. 19B is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 19A as viewed along theX direction, showing the same point in time as FIG. 19A;

FIG. 19C is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of the EUV light source of FIG.1, as viewed along the Z direction, showing a point in time during whichthe current target is interacting with a current main radiation pulse inthe target space and producing EUV light;

FIG. 19D is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 19C as viewed along theX direction, showing the same point in time as FIG. 19C;

FIG. 20A is a schematic diagram of an exemplary diagnostic system,extended target region, and target space as viewed along the Z directionof the EUV light source of FIG. 1, showing a point in time after acurrent target has interacted with three diagnostic light beams of thediagnostic system in the extended target region and during which thecurrent radiation pulse is being directed to the target space;

FIG. 20B is a schematic diagram of the exemplary diagnostic system,extended target region, and target space of FIG. 20A as viewed along theX direction, showing the same point in time as FIG. 20A;

FIG. 21A is a block diagram of an exemplary diagnostic system of the EUVlight source of FIG. 1;

FIG. 21B is an exemplary image that is recorded using a two-dimensionalrecording device of the diagnostic system of FIG. 21A;

FIG. 22 is a block diagram of a control system that can be used inconjunction with the diagnostic system of FIG. 21A for processing imagessuch as that shown in FIG. 21B;

FIG. 23 is a block diagram of an analysis module within the controlsystem of FIG. 22;

FIG. 24 is a schematic diagram of an exemplary diagnostic system inwhich shadows of a target are imaged using two or more two-dimensionalrecording devices;

FIG. 25 is a schematic diagram of an exemplary diagnostic system inwhich light scattered off a target is imaged using two or moretwo-dimensional recording devices;

FIG. 26 is a schematic diagram of an exemplary diagnostic system inwhich light reflected from a target is imaged using a two-dimensionalrecording device;

FIG. 27 is a block diagram of an exemplary diagnostic system thatrecords both one-dimensional aspects and two-dimensional images of lightproduced from the interaction between a target and diagnostic probes;and

FIG. 28 is a flow chart of an exemplary procedure performed by thecontrol system of FIG. 22.

DESCRIPTION

Referring to FIGS. 1A and 1B, an extreme ultraviolet (EUV) light source100 supplies EUV light 155 that has been produced by an interactionbetween a target and a radiation pulse to an output apparatus 160. TheEUV light source 100 includes features or components that measure andanalyze one or more moving properties (such as speed, velocity, andacceleration) of a current target 110 as the current target 110 travelsin an extended target region 115. The current target 110 travelsgenerally along a trajectory TR, the direction of which can beconsidered as a target (or axial) direction A_(T), toward a target space120 that is defined within a chamber 175. The axial direction A_(T) ofthe current target 110 lies in a three dimensional coordinate system,that is, the X, Y, Z coordinate system defined by the chamber 175. Theaxial direction A_(T) of the current target 110 generally has acomponent that is parallel with the −X direction of the coordinatesystem of the chamber 175. However, the axial direction A_(T) of thecurrent target 110 also can have components along one or more of thedirections Y and Z that are perpendicular to the −X direction.

With reference to FIGS. 1B and 2B, the EUV light source 100 adjusts oneor more characteristics of a radiation pulse 135 that is directed towardthe target space 120 based on the analysis of the determined movingproperty of the current target 110. The adjustment to the one or morecharacteristics of the radiation pulse 135 improves a relative alignmentbetween a present target 110′ and the radiation pulse 135 at the targetlocation 122 in the target space 120. The present target 110′ is thetarget that has entered the target space 120 at the time that theradiation pulse 135 (which has just been adjusted) arrives in the targetspace 120. Such adjustment to the one or more characteristics of theradiation pulse 135 improves the interaction between the present target110′ and the radiation pulse 135 and increases the amount of EUV light150 (such as shown in FIG. 1A) produced by such interaction.

In some implementations, the present target 110′ is the current target110. In these implementations, the adjustment to the one or morecharacteristics of the radiation pulse 135 happens in a relativelyshorter time frame. A relatively shorter time frame means that the oneor more characteristics of the radiation pulse 135 are adjusted duringthe time after the analysis of the moving properties of the currenttarget 110 is completed to the time that the current target 110 entersthe target space 120. Because the one or more characteristics of theradiation pulse 135 are able to be adjusted in the relatively shortertime frame, there is enough time to effect the interaction between thecurrent target 110 (the moving properties of which have just beenanalyzed) and the radiation pulse 135.

In other implementations, the present target 110′ is another target,that is, a target other than the current target 110, and following thecurrent target 110 in time. In these implementations, the adjustment tothe one or more characteristics of the radiation pulse 135 happens in arelatively longer time frame such that it is not feasible to effect theinteraction between the current target 110 (the moving properties ofwhich have just been analyzed) and the radiation pulse 135. On the otherhand, it is feasible to effect the interaction between the other (orlater) target and the radiation pulse 135. A relatively longer timeframe is a time frame that is greater than the time after the analysisof the moving properties of the current target 110 is completed to thetime that the current target 110 enters the target space 120. Dependingon the relatively longer time frame, the other target could be adjacentto the current target 110. Or, the other target could be adjacent to anintermediate target that is adjacent to the current target 110.

The EUV light source 100 is able to determine the moving property of thecurrent target 110 and each target directed toward the target space 120,and also to adjust the characteristic (or characteristics) of theradiation pulse 135 in a short window of time. Specifically, the movingproperty of the current target 110 is determined after a prior andadjacent target 110P has interacted with a prior radiation pulse 135P(FIG. 2A) but before the next target enters the extended target region115. In this way, the moving property of every or nearly every targetthat is being directed to the target space 120 can be determined so thata specific adjustment to a particular radiation pulse can be tailored tothe determined moving property of the target that the particularradiation pulse will interact with.

By measuring and analyzing the moving property of the current target 110in this extended target region 115 and in the short window of time, itis possible determine the impact or effect of various forces and effectsapplied to the current target 110 as it travels toward the target space120. For example, forces and effects that are applied to the currenttarget 110 include plasma pushback forces 125 that are applied to thecurrent target 110 due to the remaining plasma 130 that is formed froman interaction at the target location 122 within the target space 120between the prior target 110P (shown in FIG. 2A) and the prior radiationpulse 135P (shown in FIG. 2A) that is supplied by an optical source 140.Such plasma pushback forces 125 can become larger as the plasma powerincreases, and the plasma power depends on power of the prior radiationpulse 135P and the efficiency of the interaction between the priorradiation pulse 135P and the prior target 110P. Thus, it becomesimportant as these output powers are increased to account for and makeadjustments to reduce the impact of the plasma pushback forces 125.Other forces and effects applied to the current target 110 includeinstabilities in the generation and transport of the current target 110as it travels toward the target space 120 and disruptions to the targettrajectory due to the current target 110 interacting with other gas flow(such as hydrogen gas) as it travels toward the target space 120.

The current target 110 (as well as the prior target 110P and targetsreleased earlier and later than these targets) is produced by a targetdelivery system 145 and is directed toward the target space 120 along atrajectory or path TR and the current target 110 is directed along itsown axial direction A_(T) at each point along the trajectory TR. In someimplementations, the axial direction A_(T) of the current target 110upon immediate release from the target delivery system 145, aligns or isparallel with the −X direction of the three dimensional coordinatesystem X, Y, Z. The current target 110 moves at a velocity and along itsaxial direction A_(T) and such motion can be predicted based on theproperties at the target delivery system 145. Each target released bythe target delivery system 145 can have a slightly different actualtrajectory and the trajectory depends on the physical properties of thetarget delivery system 145 at the time of release of the target as wellas the environment within the chamber 175.

However, as discussed above, various forces and effects (such as theplasma pushback forces 125 applied along the X direction as well the Yand Z directions) applied to the current target 110 can cause the motionof the current target 110 to divert or change from the predicted motion.For example, the plasma pushback forces 125 can slow the current target110 (as well as the present target 110′) along the X direction or causethe current target 110 to move along the Y or Z directions in anunpredictable manner. Without taking into account the impact of theseforces and effects (such as the plasma pushback forces 125) on themovement of the present target 110′ (which can be the current target110), the radiation pulse 135 produced by the optical source 140 anddirected toward the target location 122 within the target space 120 maymiss the present target 110′ completely or may not efficiently interactwith the present target 110′ when the present target 110′ reaches thetarget location 122. This inefficient interaction can lead to areduction in the amount of EUV light 150 produced by the present target110′, and thus can lead to a reduction in the amount of EUV light 155that is output from the light source 100 toward an output apparatus 160such as a lithography exposure apparatus. Additionally, this inefficientinteraction can produce excess debris from the material of the presenttarget 110′ after it has interacted with the radiation pulse 135. Thisdebris contaminates an interior of or optics within the chamber 175, andthe contamination of the chamber interior and/or optics within thechamber 175 can force stoppage of the EUV light source 100 in order toclean the interior and/or optics or to replace optics.

The current target 110 can experience plasma pushback forces 125 thatchange its velocity (an exemplary moving property), for example, on theorder of 0.1 to 10 m/s. To resolve such a change to the velocity of thecurrent target 110, the EUV light source 100 should be able to detectchanges in the velocity to within a level that can be less than or equalto about 0.1 m/s (for example, less than or equal to about 0.04 m/s or0.02 m/s) to ensure an acceptable accuracy in a relative positionbetween the radiation pulse and the present target 110′ at the targetlocation 122, for example, a relative position of less than 5 μm.

Referring again to FIG. 1A, the extended target region 115 is thatregion in which the plasma pushback forces 125 affect the current target110 and cause the motion of the current target 110 to deviate from adesired motion. By quantifying this deviation, it is possible todetermine how to adjust the radiation pulse 135 to ensure that theradiation pulse 135 efficiently interacts with the present target 110′within the target space 120. If the present target 110′ is a targetother than the current target 110, then an assumption can be made thatthe effect of the various forces on the current target 110 is similar tothe effect of the various forces on the present target 110′ so that theanalysis can be applied to adjust the radiation pulse 135 that interactswith the target other than the current target 110.

The extended target region 115 therefore can include remaining plasma130 formed from the interaction of the prior target 110P (as shown inFIG. 2A) and the prior radiation pulse 135P (as shown in FIG. 2A). Afirst region 165 between the extended target region 115 and the targetdelivery system 145 can be considered as a region in which the plasmapushback forces 125 have a much lower effect on the current target 110.Thus, it is expected that a moving property (such as a speed ordirection) of the current target 110 in the extended target region 115will be different from the moving property of the current target 110 inthe first region 165. Such a difference may make it difficult toefficiently interact the radiation pulse 135 with the present target110′ when it reaches the target location 122 within the target space 120because the present target 110′ may arrive at a different location thanplanned within the target space 120 and thus the radiation pulse 135 maynot fully or partly intercept the present target 110′.

In order to measure the moving property of the current target 110, theEUV light source 100 includes a diagnostic system 105 that provides oneor more diagnostic probes 107 that interact with the current target 110in the extended target region 115, as shown in FIG. 1A. Specifically,the one or more diagnostic probes 107 interact with the current target110 in the extended target region 115 only after the prior and adjacenttarget 110P has already interacted with the prior radiation pulse 135Pin the target space 120. The one or more diagnostic probes 107 can bedirected along a direction that is in a plane of the −X direction andthe −Y direction, for example, along the −Y direction. Moreover, the oneor more diagnostic probes 107 can be configured to interact with eachand every target 110 that passes through the extended target region 115so that the diagnostic system 105 analyzes information about each andevery target 110.

The interaction between the current target 110 and the one or morediagnostic probes 107 releases information (such as light or photons)that can be detected by the diagnostic system 105. The diagnostic system105 outputs data based on the released information, and that data can beused to determine the moving property of the current target 110. The EUVlight source 100 also includes a control system 170 that receives thisdata from the diagnostic system 105. The control system 170 analyzesthis data and determines the moving property of the current target 110based on this analysis.

The EUV light source 100 performs the measurement and analysis on themoving property of the current target 110 in the extended target region115 and also makes a change to one or more characteristics of theradiation pulse 135 that will interact with the present target 110′ atthe target location 122 within the target space 120 so that the presenttarget 110′ and the radiation pulse 135 efficiently interact with eachother to produce EUV light 150. The radiation pulse 135 that interactswith the present target 110′ at the target location 122 within thetarget space 120 may or may not be the very next radiation pulse that isproduced by the optical source 140 after the production of the priorradiation pulse 135P.

The time frame during which the EUV light source 100 performs themeasurement and analysis as well as the adjustment or change to theradiation pulse 135 is constrained by one or more of the rate at whichthe target delivery system 145 generates and releases each target alongthe trajectory TR and a distance between the target delivery system 145and the target space 120. For example, if the target delivery system 145generates targets at a repetition rate of 50 kHz, and a velocity of atarget is 70 meters per second (m/s) as it is released from the targetdelivery system 145, then each target in the trajectory TR is physicallyseparated or spaced by about 1.4 millimeters (mm) along the trajectoryTR. Given these exemplary conditions, each target crosses the path ofthe diagnostic probe(s) 107 of the diagnostic system 105 every 20microseconds (μs). In this example, the EUV light source 100 mustperform the measurement and analysis on the current target 110 as wellas affect the change to the radiation pulse 135 all within a time frameof 20 μs just after the prior target 110P and the prior radiation pulse135P interact, and also within distances that are less than the spacingbetween the targets (which would be 1.4 mm in this example).

The plasma pushback forces 125 extend out from the target space 120 andthe size of the forces drop with the distance from the target space 120.For example, the plasma pushback forces 125 can drop with a linearmultiple of the distance or with a square of the distance. For example,the plasma pushback forces 125 generated within the target space 120 canaffect the current target 110 as far out as 1.0 to 1.5 mm or even up to10 mm from the target space 120 along any of the directions, and forexample, along the X direction. By contrast, the distance between thetarget space 120 and the target delivery system 145 is about 1 meter(m).

The EUV light source 100 includes the chamber 175 that defines thetarget space 120, the first region 165, and the extended target region115, which is closer to the target space 120 than the first region 165,all within the three dimensional coordinate system X, Y, Z. The targetdelivery system 145 is configured to release the current target 110along the trajectory or path TR that overlaps both the first region 165and the extended target region 115. As discussed above, the targetdelivery system 145 releases a stream of targets at a particular rate,and the EUV light source 100 must take this rate into account whendetermining the total amount of time needed to perform the measurementand analysis on the moving property (or properties) of the currenttarget 110 as well as affecting a change to the radiation pulse 135 thatinteracts with the present target 110′ at the target location 122 withinthe target space 120.

The EUV light source 100 includes a light collector 180 that collects asmuch EUV light 150 emitted from the plasma as possible and redirectsthat EUV light 150 as collected EUV light 155 toward the outputapparatus 160.

The EUV light source 100 includes a beam delivery system 185 thatdirects the beam of radiation pulse or pulses 135P, 135 from the opticalsource 140 to the target space 120 and generally along the Z direction(though the beam or beams 135, 135P can be at an angle relative to the Zdirection). The beam delivery system 185 can include optical steeringcomponents 185A that change a direction or angle of the beam ofradiation pulses 135, 135P and a focus assembly 185B that focuses thebeam of radiation pulses 135, 135P to the target space 120. Exemplaryoptical steering components 185A include optical elements such as lensesand mirrors that steer or direct the beam of radiation pulses byrefraction or reflection, as needed. The beam delivery system 185 canalso include an actuation system that controls or moves the variousfeatures of the optical components 185A and the focus assembly 185B.

Each of the targets (such as the present target 110′ the current target110, the prior target 110P, and all other targets produced by the targetdelivery system 145) includes a material that emits EUV light whenconverted to plasma. Each target is converted at least partially ormostly to plasma through interaction with the radiation pulse 135produced by the optical source 140 at the target location 122 within thetarget space 120.

Each target (including the current target 110 and the prior target 110P)produced by the target delivery system 145 is a target mixture thatincludes a target substance and optionally impurities such as non-targetparticles. The target substance is the substance that is capable ofbeing converted to a plasma state that has an emission line in the EUVrange. The target substance can be, for example, a droplet of liquid ormolten metal, a portion of a liquid stream, solid particles or clusters,solid particles contained within liquid droplets, a foam of targetmaterial, or solid particles contained within a portion of a liquidstream. The target substance can include, for example, water, tin,lithium, xenon, or any material that, when converted to a plasma state,has an emission line in the EUV range. For example, the target substancecan be the element tin, which can be used as pure tin (Sn); as a tincompound such as SnBr4, SnBr2, SnH4; as a tin alloy such as tin-galliumalloys, tin-indium alloys, tin-indium-gallium alloys, or any combinationof these alloys. In the situation in which there are no impurities, theneach target includes only the target substance. The discussion providedherein is an example in which each target is a droplet made of moltenmetal such as tin. However, each target produced by the target deliverysystem 145 can take other forms.

The current target 110 can be provided to the target space 120 bypassing molten target material through a nozzle of the target deliverysystem 145, and allowing the current target 110 to drift into the targetspace 120. In some implementations, the current target 110 can bedirected to the target space 120 by force. The current target 110 can bea material that has already interacted with one or more radiation pulses135 or the current target 110 can be a material that has not yetinteracted with one or more radiation pulses 135.

The optical source 140 is configured to produce a plurality of radiationpulses that are directed toward the target space 120 by way of the beamdelivery system 185. Each radiation pulse that interacts with a targetat the target location 122 within the target space 120 converts at leasta part of that target into plasma that emits EUV light 150.

The EUV light source 100 also includes an adjustment system 190 coupledto the optical source 140 and to the control system 170. The controlsystem 170 is configured to control a relative position between aradiation pulse 135 and the present target 110′ by sending a controlsignal to the adjustment system 190. The control signal causes theadjustment system 190 to adjust one or more of a timing of a release ofthe radiation pulse 135 and a direction at which the radiation pulse 135travels.

Referring to FIG. 3, an exemplary diagnostic system 305 is shown. Thediagnostic system 305 includes a probe module 300 which can be anillumination module 300 that produces, under control of the controlsystem 170 or control system 470 (discussed below), as the diagnosticprobe 107 at least two diagnostic light beams 320, 330 that are directedtoward the trajectory TR of the current target 110. As discussed above,the diagnostic probe 107 (in this case, the diagnostic light beams 320,330) interacts with the current target 110 in the extended target region115. Accordingly, the diagnostic light beam 320 is directed to interactwith the current target 110 at a location 322 and a time T₃₂₀ in theextended target region 115, and the diagnostic light beam 330 isdirected to interact with the current target 110 at a location 328 andat a time T₃₃₀ in the extended target region 115. The time T₃₃₀ is afterthe time T₃₂₀. The diagnostic light beams 320, 330 form laser curtainsthrough which the current target 110 traverses. In some implementations,such as shown in FIG. 3, the diagnostic light beams 320, 330 can bedirected along a path that crosses the trajectory TR at a right angle(an angle of approximately 90°) to the −X direction.

Moreover, the diagnostic light beams 320, 330 are separated from eachother along the X direction by a known distance, for example, a valuethat can be referred to as Δd. For example, separation Δd can be lessthan the spacing between the targets and it can be determined or setbased on the spacing between the targets to provide for greaterprecision in the measurements that are performed based on theinteractions between the diagnostic light beams 320, 330 and the currenttarget 110. Up to a point and in general, the larger the separation Δdthe higher the precision in the measurements that are performed. Forexample, the separation Δd can be between about 250 μm and 800 μm.

The interactions between the diagnostic light beams 320, 330 and thecurrent target 110 enable the control system 170 or 470 to determine amoving property such as a velocity V of the current target 110 along the−X direction. It is possible to determine trends in the velocity V orthe changing velocity V over many targets. It is also possible todetermine a change in a moving property of the current target 110 alongthe −X direction using only the diagnostic light beams 320, 330 if someassumptions about the motion of the current target 110 are made.

In some implementations, the illumination module 300 includes a singlelight source that produces a light beam that is split into twodiagnostic light beams (such an exemplary design is shown in FIG. 5).For example, a single light source can be a solid-state laser such as aYAG laser, which can be a neodymium-doped YAG (Nd:YAG) laser operatingat 1070 nm and at 50 W power. In this example, the illumination module300 also includes one or more optical elements (such as a beam splitteror mirrors) that split the light beam from the YAG laser into twoseparate diagnostic light beams that are directed toward the trajectoryTR of the target 110 as diagnostic light beams 320, 330. In otherimplementations, the illumination module 300 includes a pair of lightsources such as two lasers, each producing its own diagnostic light beam320, 330.

The diagnostic system 305 also includes a detection module 335. Thedetection module 335 is configured to detect the data that results fromthe interaction between the current target 110 and the respectivediagnostic light beam 320, 330 within the extended target region 115,and then output the detected data to the control system 170 or 470. Forexample, the detection module 335 can detect each interaction bydetecting a one-dimensional aspect or characteristic such as theintensity of the light 340, 350 that is reflected from the currenttarget 110 as the respective diagnostic light beam 320, 330 strikes thetarget 110. Moreover, the control system 170 or 470 can analyze the datafrom the detection module 335 and, based on the analysis, detect thetime at which the maximum intensity of the light 340, 350 that isreflected from the current target 110 reaches the detection module 335.The light 340, 350 that is reflected from the current target 110 can bea portion of the respective diagnostic light beam 320, 330 that isreflected from the current target 110. The accuracy with which the EUVlight source 100 can detect the changes to the trajectory of the currenttarget 110 is limited to the resolution of the detection module 335.

In some implementations, the detection module 335 includes a photodetector and one or more optical components such as reflective orrefractive optics, filters, apertures to direct and modify the light340, 350 prior to entering the photo detector.

The wavelength of the diagnostic probe (and the diagnostic light beams320, 330) produced by the illumination module 300 should be distinctenough from the wavelength of the radiation pulses 135 produced by theoptical source 140 so that the detection module 335 can distinguishbetween the light 340, 350 reflected from the current target 110 andstray light from the radiation pulses 135. In some implementations, thewavelength of the diagnostic light beams 320, 330 is 532 nm or 1550 nm.

It is also possible that the diagnostic system 105, 305 includes anoptic that changes a polarization state of one or more of the diagnosticlight beams 320, 330.

In some implementations, the diagnostic light beams 320, 330 produced bya laser source are Gaussian beams, and thus the transverse profile ofthe optical intensity of each diagnostic light beam 320, 330 can bedescribed with a Gaussian function. In such a function, the opticalintensity correlates with the transverse distance from the axis of thelight beam 320 or 330. The transverse profile of the diagnostic lightbeam 320, 330 also determines how the detection module 335 measures thelight 340, 350 reflected from the current target 110 because thedifferent transverse profiles of the diagnostic light beam 320, 330 canalter one or more aspects of the light 340, 350 detected by thedetection module 335. The transverse profile of the diagnostic lightbeam 320 or 330 could be used to determine a moving property of thecurrent target 110 that has a component in the Y direction if thediagnostic light beam 320, 330 were to be directed along a path thatsubtends a non-right angle with the trajectory TR of the current target110, such as shown in FIG. 7.

The control system 170 or 470 is configured to analyze the data outputfrom the diagnostic system 105, 305 and control a relative positionbetween the radiation pulse 135 and the present target 110′ based on theanalysis. To this end, and with reference to FIG. 4, an exemplarycontrol system 470 includes a detection sub-controller 400 that receivesthe output from the diagnostic system 305. The detection sub-controller400 analyzes the output from the detection module 335 of the diagnosticsystem 305, and determines one or more moving properties of the currenttarget 110 based on this analysis. The detection sub-controller 400 alsodetermines whether an adjustment needs to be made to the radiation pulse135 output from the optical source 140 based on this determination; andif an adjustment is needed, the detection sub-controller 400 sends anappropriate signal to an optical source sub-controller 405, whichinterfaces with the optical source 140.

In some implementations, the detection module 335 of the diagnosticsystem 305 outputs a one-dimensional signal such as a voltage signalthat is generated when photons of the light 340, 350 are detected. Thus,the detection module 335 detects a one-dimensional aspect (such as thephotons) of the light 340, 350. The detection sub-controller 400converts the output (such as the voltage signal) from the detectionmodule 335 into a value associated with the light 340 produced from theinteraction between the current target 110 and the diagnostic light beam320, and a value associated with the light 350 produced from theinteraction between the current target 110 and the diagnostic light beam330. These two values can be used to determine the one or more movingproperties of the target 110.

For example, the detection sub-controller 400 can convert the voltagesignal from the detection module 335 into a first value that correspondsto a maximum intensity of the light 340 produced from the interactionbetween the current target 110 and the diagnostic light beam 320, and asecond value that corresponds to the maximum intensity of the light 350produced from the interaction between the current target 110 and thediagnostic light beam 330. These two values of the maximum intensity canbe digitally time stamped and then used to determine the one or moremoving properties of the target 110, as discussed below in greaterdetail.

The sub-controller 400 can include a field-programmable hardware circuit400A, such as a field-programmable gate array (FPGA), which is anintegrated circuit designed to be configured by a customer or a designerafter manufacturing. The circuit 400A can be dedicated hardware thatreceives the values of the time stamps from the detection module 335,performs a calculation on the received values, and uses one or morelookup tables to estimate a time of arrival of the present target 110′at the target location 122. In particular, the circuit 400A can be usedto quickly perform a calculation to enable the adjustment to the one ormore characteristics of the radiation pulse 135 in the relativelyshorter time frame to enable the adjustment of the one or morecharacteristics of the radiation pulse 135 that interacts with thecurrent target 110, the moving properties of which have just beenanalyzed by the circuit 400A.

For example, the circuit 400A can perform a subtraction step on the timestamps to determine the value of the difference ΔT. The circuit 400Aaccesses the stored the values of the separation Δd, and the value ofthe distance D_(RB2) between the crossing of the diagnostic light beam330 with the trajectory TR of the current target 110 and the targetlocation 122 along the X direction. The circuit 400A can thereforerapidly perform a calculation using a simple and fast technique thatdoes not require the use of other software within the sub-controller 400or within other components of the control system 470. For example, thecircuit 400A can access a flight time lookup table that stores a set ofvelocities V for specific values of the difference ΔT given the value ofthe separation Δd, and a set of times of arrival to the target location122 that correlate with various values of D_(RB2) divided by velocity Vto quickly output the time of arrival to the sub-controller 400, for useby other components of the control system 470.

The control system 470 also includes a sub-controller 410 specificallyconfigured to interface with the beam delivery system 185, asub-controller 412 specifically configured to interface with the probemodule 300, and a sub-controller 415 specifically configured tointerface with the target delivery system 145. Moreover, the controlsystem 470 can include other sub-controllers specifically configured tointerface with other components of the light source 100 not shown inFIG. 1.

The control system 470 generally includes one or more of digitalelectronic circuitry, computer hardware, firmware, and software. Thecontrol system 470 can also include memory 450, which can be read-onlymemory and/or random access memory. Storage devices suitable fortangibly embodying computer program instructions and data include allforms of non-volatile memory, including, by way of example,semiconductor memory devices, such as EPROM, EEPROM, and flash memorydevices; magnetic disks such as internal hard disks and removable disks;magneto-optical disks; and CD-ROM disks. The control system 470 can alsoinclude one or more input devices 455 (such as a keyboard, touch screen,microphone, mouse, hand-held input device, etc.) and one or more outputdevices 460 (such as speakers and monitors).

The control system 470 includes one or more programmable processors 465,and one or more computer program products 467 tangibly embodied in amachine-readable storage device for execution by a programmableprocessor (such as the processors 465). The one or more programmableprocessors 465 can each execute a program of instructions to performdesired functions by operating on input data and generating appropriateoutput. Generally, the processor 465 receives instructions and data frommemory 450. Any of the foregoing may be supplemented by, or incorporatedin, specially designed ASICs (application-specific integrated circuits).

Moreover, any one or more of the sub-controllers 400, 405, 410, 412, 415can include their own digital electronic circuitry, computer hardware,firmware, and software as well as dedicated memory, input and outputdevices, programmable processors, and computer program products.Likewise, any one or more of the sub-controllers 400, 405, 410, 412, 415can access and use the memory 450, the input devices 455, the outputdevices 460, the programmable processors 465, and the computer programproducts 467.

Although the control system 470 is shown as a separate and completeunit, it is possible for each of the components and sub-controllers 400,405, 410, 412, 415 to be separate units within the light source 100.

Referring to FIG. 5, an exemplary diagnostic system 505 is shown ashaving a probe module such as an illumination module 500 that includes asingle light source 502 that produces, under control of the controlsystem 170, 470, 670, a light beam 510, a set of optical components 515,517, and a pair of diagnostic light beams 520, 530 that serve as thediagnostic probe 107. The optical components 515, 517 of the set areconfigured and designed to split the light beam 510 into the twodiagnostic light beams 520, 530 as well as direct the diagnostic lightbeams 520, 530 toward the trajectory TR of the current target 110. Insome examples, the optical component is a beam splitter 515 that splitsthe light beam 510 into diagnostic light beams 520, 530. For example,the beam splitter 515 can be a dielectric mirror, a beam splitter cube,or a polarizing beam splitter. One or more optical components 517 suchas reflective optics can be placed to redirect either or both of thediagnostic light beams 520, 530 so that both diagnostic light beams 520,530 are directed toward the trajectory TR of the current target 110. Theset of optical components 515, 517 can include other optical componentsnot shown or in a different configuration from what is shown.

The diagnostic system 505 includes the detection module 535, which isconfigured to detect the light 540, 550 reflected from the currenttarget 110 as the respective diagnostic light beam 520, 530 strikes thetarget 110. The detection module 535 can include a device such as aphotodiode that converts the light (in the form of photons) into acurrent, and outputs a voltage that is related to the current. Thus, inthis example, the output from the detection module 535 constitutes aone-dimensional voltage signal, which is output to the control system670. The detection module 535 can also include optical filters,amplifiers, and built-in lenses, as needed. The photodiode generates thecurrent when photons from the light 540, 550 are absorbed in thephotodiode and outputs a voltage signal that corresponds to thegenerated current. The detection module 535 generates as the voltagesignal an analog pulse 560 when the light 540 is detected and an analogpulse 570 when the light 550 is detected. These pulses 560, 570 areoutput from the detection module 535 to the control system 670 forfurther processing.

As shown, the detection module 535 includes a single device such as aphotodiode detector that is able to detect both of the interactions(that is, both the light 540, 550). Such a design that uses a singledevice reduces complexity and also enables the data to be moreefficiently analyzed. In other implementations, the detection module 535includes one or more photo-transistors, light-dependent resistors, andphotomultiplier tubes. In other implementations, the detection module535 includes one or more thermal detectors such as a pyroelectricdetector, a bolometer, or a calibrated charged coupled device (CCD) orCMOS.

Referring to FIG. 6, an exemplary control system 670 is shown forprocessing the output from the diagnostic system 505 to determine avalue of the velocity (moving property) of the current target 110 alongthe X direction. The exemplary control system 670 includes a detectionsub-controller 600 that receives the pulses 560, 570 from the diagnosticsystem 505. The detection sub-controller 600 includes a discriminatormodule 605 that receives the pulses 560, 570 and filters this signal,amplifies this signal, and differentiates it, as needed. At azero-crossing of the derivative of each current target 110 signal(generated from the pulses 560, 570), the discriminator module 605generates a digital trigger pulse 610, 620, respectively. Thediscriminator module 605 can be an electrical circuit that includes afilter and a gain circuit as well as a peak predict circuit withdifferentiation capabilities.

The detection sub-controller 600 also includes a time module 625 thatreceives the digital trigger pulses 610, 620 and digitally time stampseach individual trigger pulse 610, 620 as T₅₂₀ and T₅₃₀. The differencebetween the time stamps T₅₂₀ and T₅₃₀ is given as ΔT. The detectionsub-controller 600 includes a moving property module 635 to which thevalue of ΔT is input. Thus, the detection sub-controller 600 convertsthe signals associated with the respective light 540, 550 reflected fromthe current target 110 into respective single data values such as timestamps that can be used for further analysis.

The moving property module 635 also accesses the value of Δd from memory450, which can be internal to or external to the moving property module635. The moving property module 635 determines the value of the velocityof the current target 110 in the extended target region 115. Forexample, the moving property module 635 could use the determined valueof ΔT and the value of Δd, and compare those values to a set ofpre-determined values stored in memory such as memory 450 to determine avalue of the velocity of the current target 110. As another example, themoving property module 635 could calculate the average velocity V of thecurrent target 110 along the X direction as Δd/ΔT.

The moving property module 635 can also estimate or determine anacceleration of the current target 110 if assumptions are made about themotion of the current target 110. It is possible to determine trends inthe velocity V or the changing velocity V over many targets.

The moving property module 635 also determines the predicted time thatthe present target 110′ (which can be the current target 110) will be atthe target location 122 within the target space 120. The moving propertymodule 635 is able to determine the predicted time of arrival of thecurrent target 110 at the target location 122 because the value of thevelocity V of the current target 110 has been determined as well asother information about the current target 110 and the diagnosticradiation beam 530 relative to the target location 122. Specifically,the moving property module 635 knows the distance D_(RB2) between thecrossing of the diagnostic light beam 530 with the trajectory TR of thecurrent target 110 and the target location 122 along the X direction.The moving property module 635 also knows the time that the currenttarget 110 passed through the path of the diagnostic light beam 530.Thus, it is possible to estimate or determine the arrival of the currenttarget 110 at the target location 122 as being the distance D_(RB2)divided by the velocity V (or D_(RB2)/V).

The output from the moving property module 635 is a control signal andis directed to the optical source sub-controller 405, which interfaceswith the adjustment system 190 coupled to the optical source 140. Thecontrol signal from the moving property module 635 provides instructionsthat cause the adjustment system 190 to adjust aspects of the opticalsource 140 to thereby adjust one or more of a timing of a release of theradiation pulse 135 and a direction at which the radiation pulse 135travels.

Referring to FIG. 7, in other implementations, an exemplary diagnosticsystem 705 includes an illumination module 700 that produces as thediagnostic probe 107 three diagnostic light beams 720, 725, 730. Thediagnostic light beams 720, 725, 730 are directed toward respectivelocations 722, 724, 728 along the trajectory TR of the current target110 to interact with the current target 110 at respective times T₇₂₂,T₇₂₄, T₇₂₈. The respective interactions between the diagnostic lightbeams 720, 725, 730 and the current target 110 produce light 740, 745,750. The diagnostic system 705 therefore includes the detection module735, which is configured to detect the light 740, 745, 750 reflectedfrom the current target 110 as the respective diagnostic light beam 720,725, 730 interacts with the current target 110. The detection module 735can include a device such as a photodiode that converts the light intocurrent. The diagnostic system 705 can be coupled to a control system870, which is a specific implementation of the control system 170 andwill be discussed with reference to FIG. 8.

By including a third diagnostic light beam 725, it is possible todetermine not only a moving property such as velocity V of the currenttarget 110 along the −X direction, but also to determine a change in themoving property of the current target 110 along the −X direction. Thus,the use of the third diagnostic light beam 725 enables the controlsystem 170 to determine both the velocity V and the acceleration A ofthe current target 110 along the −X direction.

Additionally, because the third diagnostic light beam 725 is directedtoward the trajectory TR at a non-right angle relative to the trajectoryTR, the control system 870 is able to determine one or more movingproperties (such as the velocity or trajectory) of the current target110 along a direction that is perpendicular to the −X direction, forexample along the Y direction, as discussed below.

The diagnostic light beams 720, 730 are directed along a path thatcrosses the trajectory TR of the current target 110 at a right (90°) orapproximately right angle relative to the −X direction. The diagnosticlight beam 725 is directed along a path that crosses the trajectory TRof the current target 110 at a non-right angle (for example, at an angleof approximately 45°) relative to the −X direction. Thus, the diagnosticlight beams 720, 730 generally travel along the −Y direction while thediagnostic light beam 725 travels along a direction in a plane definedby the X and Y (generally along −Y and either −X or X directions).

As discussed above, the diagnostic light beams 720, 725, 730 interactwith the current target 110 as the current target 110 travels toward thetarget space 120 and while in the extended target region 115. Thediagnostic light beams 720, 725, 730 are separated from each other alongthe X direction by known distances, as discussed below, and this knowninformation can be used to determine one or more moving properties ofthe current target 110. For example, the velocity and acceleration ofthe current target 110 along the −X direction can be determined.Additionally, information about a displacement or motion along the Ydirection can also be determined.

Referring to FIG. 8, an exemplary detection sub-controller 800 can bedesigned as a part of the control system 870 in order to analyze thedata obtained from the interaction between the diagnostic system 705 andthe current target 110. For example, the detection sub-controller 800receives pulses 760, 765, 770 output from the diagnostic system 705. Thepulses 760, 765, 770 correspond to the analog pulses produced by thedetection module 735 when respective light 740, 745, 750 is detected.

The distance between the diagnostic light beams 720, 730 along the Xdirection is known and can be denoted as Δd1(X). In one example, theseparation Δd1(X) is 100 μm. Thus, the diagnostic light beams 720, 730can be used by the control system 870 to determine the velocity V1 ofthe current target 110 along the −X direction in the extended targetregion 115 using, for example, the method discussed above with respectto FIGS. 5 and 6. Specifically, control system 170 determines timestamps T₇₂₂ and T₇₂₈ associated with the light 740, 750 produced fromthe interaction between the respective diagnostic light beams 720, 730and the current target 110 at respective locations 722, 728 along thetrajectory TR. The control system 870 calculates the difference betweenthese time stamps ΔT1(X). The control system 870 determines the value ofthe velocity V1 of the current target 110 along the −X direction in theextended target region 115 based on the determined values of ΔT1(X) andΔd1(X). For example, the control system 870 can calculate the velocityV1 of the current target 110 along the X direction as Δd1(X)/ΔT1(X).

Additionally, the control system 870 determines a time stamp T₇₂₄associated with the light 745 produced from the interaction between thediagnostic light beam 725 and the current target at the location 724along the trajectory TR. The distance along the −X direction between thediagnostic light beams 720 and 725 at the locations 722, 724 is knownand can be denoted as Δd2(X). The distance along the −X directionbetween the diagnostic light beams 725 and 730 at the locations 724, 728is also known and can be denoted as Δd3(X). Using this additionalinformation, the control system 870 can calculate a time differenceΔT2(X) between the time stamps T₇₂₄ and T₇₂₂ and a time differenceΔT3(X) between the time stamps T₇₂₈ and T₇₂₄. The control system 870 cantherefore determine the velocity V2 of the current target along the −Xdirection as it travels between location 722 and 724 as Δd2(X)/ΔT2(X),and the velocity V3 of the current target along the −X direction as ittravels between location 724 and 728 as Δd3(X)/ΔT3(X).

The diagnostic light beam 725 can be used in combination with one ormore of the diagnostic light beams 720, 730 to determine a change in amoving property (for example, an acceleration A) of the current target110 along the −X direction. Specifically, the control system 870determines the time stamp T₇₂₄ associated with the light 745 producedfrom the interaction of the diagnostic light beam 725 and the currenttarget 110 at the location 724. In this way, the velocity V2(X) can bedetermined for the current target 110 between the diagnostic light beam720 and the diagnostic light beam 725 based on a difference ΔT2(X)between the time stamps T₇₂₂ and T₇₂₄ and a distance Δd2(X) between thelocations 722 and 724. Moreover, the velocity V3(X) can be determinedfor the current target 110 between the diagnostic light beam 725 and thediagnostic light beam 730 based on a difference ΔT3(X) between the timestamps T724 and T728 and a distance Δd3(X) between the locations 724 and728. The difference between these two velocities (V2(X)−V3(X)) can bedivided by the time difference to obtain the acceleration of the currenttarget 110 along the −X direction. For example, it can be assumed thatthe current target 110 has the velocity V2(X) at time T₇₂₄ and thevelocity V3(X) at time T₇₂₈ and thus the acceleration A can bedetermined to be (V2(X)−V3(X))/(T₇₂₄−T₇₂₈).

As discussed above, the diagnostic light beams 720, 725, 730 produced bythe laser source within the illumination module 700 can be Gaussianbeams. In this case, the transverse profile of the optical intensity ofeach diagnostic light beam 720, 725, 730 can be described with aGaussian function. In such a function, the optical intensity correlateswith the transverse distance from the axis of the light beam 720, 725,or 730. Because the Gaussian shape is relatively simple, this particularaspect of the diagnostic light beam 725 can be used to process dataobtained from the interaction between the diagnostic light beams 720,725, 730 and the current target 110.

The diagnostic light beam 725 can be used by the control system 870 todetermine a trajectory of the current target 110, specifically, todetermine a distance or a velocity that the current target 110 travelsalong the Y direction. This can be determined because the diagnosticlight beam 725 is directed at an angle in a plane defined by the X and Ydirections.

As shown in FIG. 9A, the diagnostic light beam 720 crosses thetrajectory TR at the location 722. The diagnostic light beam 720 travelsalong a direction defined by its axis 920A, which generally aligns withthe −Y direction. In FIG. 9A, the current target 110 generally alignswith the X direction (is at Y=0) and thus the current target 110 doesnot have a measurable Y direction component to it. By contrast, in FIG.9B, the current target 110 is offset from the X direction along the −Ydirection by an amount dY. However, because this offset still alignswith the axis 920A of the diagnostic light beam 720, the reflected light740 from the current target will not change by a significant amount.Moreover, the time at which the reflected light 740 is detected in bothexamples (FIGS. 9A and 9B) is the same or nearly the same because theinteraction between the target 110 and the diagnostic light beam 720occurs at approximately the same time. It is noted that the intensity ofthe diagnostic light beam 720 does change by an amount depending on thedistance from the beam waist, but that change may not significant enoughto be measurable or to show up as a change in the intensity of thereflected light 740.

By contrast, as shown in FIG. 9C, the diagnostic light beam 725 crossesthe trajectory TR at the location 724C and the current target 110interacts with the diagnostic light beam 725 at the time T_(724C). Inthis case, the diagnostic light beam 725 travels along a direction thatis in the XY plane and its axis 925A has components in both the X and Ydirections. Thus, the intensity of the beam 725 decreases according tothe Gaussian function along both the X and Y directions. The currenttarget 110 aligns with the −X direction and does not have anyappreciable motion along the Y direction. By contrast, as shown in FIG.9D, the current target 110 is shifted along the Y direction by thedistance dY. In FIG. 9D, the diagnostic light beam 725 is directed suchthat its axis 925A has components in both the X and Y directions, andthe offset current target 110 would be interacting with the highestintensity of the light beam 725 at a different location 724D and also ata time T_(724D), which is later than the time T_(724C). Therefore, thedetection module 735 detects the reflected light 745D in FIG. 9D at alater time than it would detect the reflected light 745C in FIG. 9C.This difference in time at which the reflected light 745C or 745D isdetected by the detection module 735 can be used to determine how farthe current target 110 has shifted along the Y direction.

Specifically, if the time difference ΔT2(X) for a current target 110 isgreater than the time difference ΔT2(X) for a prior target 110P thenthis means that the current target 110 has moved along the Y directionrelative to the prior target 110P. By contrast if the time differenceΔT2(X) for a current target 110 is less than the time difference ΔT2(X)for a prior target 110P then this means that the current target 110 hasmoved along the −Y direction relative to the prior target 110P.

Referring to FIG. 10, in other implementations, an exemplary diagnosticsystem 1005 includes an illumination module 1000 that includes a singlelight source 1002 that produces a light beam 1010. The diagnostic system1005 produces a plurality of diagnostic light beams 1020, 1025, 1030that serve as the diagnostic probe or probes 107. To this end, theillumination module 1000 also includes a diffractive optic 1015 and arefractive optic 1017 such as a focusing lens. The light beam 1010 isdirected through the diffractive optic 1015, which splits the light beam1010 into a plurality of light beams, which travel along distinctdirections and are directed through the refractive optic 1017 to producethe diagnostic light beams 1020, 1025, 1030. The diagnostic light beams1020, 1025, 1030 are directed toward the trajectory TR of the currenttarget 110. The diffractive optic 1015 can split the light beam 1010 sothat the diagnostic light beams 1020, 1025, 1030 are separated by a setdistance (for example, 0.65 mm) at the trajectory TR. Moreover, therefractive optic 1017 can ensure that the foci (or beam waist) of eachof the diagnostic light beams 1020, 1025, 1030 overlaps the trajectoryTR.

Because of the design of the diffractive optic 1015 and the refractiveoptic 1017, the diagnostic light beams 1020, 1025, 1030 are directed sothat they fan out toward the trajectory TR and intersect the trajectoryTR at different and distinct angles. For example, the diagnostic lightbeam 1025 can intersect the trajectory TR at a right or approximatelyright angle to the −X direction. The diagnostic light beam 1020 canintersect the trajectory TR at an angle that is less than 90° relativeto the −X direction and the diagnostic light beam 1030 can intersect thetrajectory TR at an angle that is greater than 90° relative to the −Xdirection. Each of the diagnostic light beams 1020, 1025, 1030 can beGaussian beams so that the transverse profile of the optical intensityof each diagnostic light beam 1020, 1025, 1030 can be described with aGaussian function. The beam waist of each diagnostic light beam 1020,1025, 1030 can be configured to overlap at the trajectory TR or the −Xdirection.

The diffractive optic 1015 can be a rectangular or binary phasediffraction grating that produces discrete and spatially spaced replicasof the input light beam 1010. The separation between the diagnosticlight beams 1020, 1025, 1030 can be adjusted or customized depending onthe rate at which the targets are released from the target deliverysystem 145 as well as the size and material of the targets. It is alsopossible to produce more than three diagnostic light beams 1020, 1025,1030 with the diffractive optic 1015. By producing so many diagnosticlight beams, it is possible to record or detect the position of thecurrent target 110 through the extended target region 115, thus allowingfor a more accurate determination of the velocity and acceleration ofthe current target 110 and also providing a tool for understanding thedynamics of the current target 110 as a result of the plasma pushbackforces 125.

In some implementations, the diffractive optic 1015 is a binary phasediffraction grating.

The diagnostic system 1005 also includes a detection module 1035 thatreceives the light 1040, 1045, 1050 reflected from the current target110 as it passes across the respective diagnostic light beams 1020,1025, 1030. The detection module 1035 can include a detection devicethat converts the photons of the light 1040, 1045, 1050 into a current,and outputs a one-dimensional voltage signal based on the current. Forexample, the detection module 1035 can include a photon detection devicesuch as a photodiode that converts the light 1040, 1045, 1050 into anelectrical signal.

Referring to FIG. 11, in some implementations, the present target 110′interacts with two radiation pulses within the target space 120. Forexample, the optical source 140 can be configured to supply apreliminary radiation pulse 1135A to a first target location 1122Awithin a target space 1120 and a main radiation pulse 1135B to a secondtarget location 1122B within the target space 1120. The radiation pulses1135A, 1135B can be directed along the Z direction.

The interaction between the preliminary radiation pulse 1135A and thepresent target 1110′ at the first target location 1122A causes thepresent target 1110′ to modify its shape so as to deform and togeometrically expand as it moves through the target space 1120. Theinteraction between the main radiation pulse 1135B and the modifiedpresent target 1110′ at the second target location 1122B converts atleast part of the modified present target 1110′ into plasma 1130 thatemits EUV light 1150. It is possible for some of the material of thepresent target 1110′ to be converted into plasma when it interacts withthe preliminary radiation pulse 1135A. However, the properties of thepreliminary radiation pulse 1135A are selected and controlled so thatthe predominant action on the present target 1110′ by the preliminaryradiation pulse 1135A is the deformation and modification of thegeometric distribution of the present target 1110′.

The interaction between the preliminary radiation pulse 1135A and thepresent target 1110′ causes material to ablate from the surface of thepresent target 1110′ and this ablation provides a force that deforms thepresent target 1110′ so that it has a shape that is different than theshape of the present target 1110′ prior to interaction with thepreliminary radiation pulse 1135A. For example, prior to interactingwith the preliminary radiation pulse 1135A, the present target 1110′ canhave a shape that is similar to a droplet upon exiting the targetdelivery system 145, while after interaction with the preliminaryradiation pulse 1135A, the shape of the present target 1110′ deforms sothat its shape is closer to the shape of a disk (such as a pancakeshape) when it reaches the second target location 1122B. Afterinteraction with the preliminary radiation pulse 1135A, the presenttarget 1110′ can be a material that is not ionized (a material that isnot a plasma) or that is minimally ionized. After interaction with thepreliminary radiation pulse 1135A, the present target 1110′ can be, forexample, a disk of liquid or molten metal, a continuous segment oftarget material that does not have voids or substantial gaps, a mist ofmicro- or nano-particles, or a cloud of atomic vapor.

Additionally, the interaction between the preliminary radiation pulse1135A and the present target 1110′ that causes the material to ablatefrom the surface of the present target 1110′ can provide a force thatcan cause the present target 1110′ to acquire some propulsion or speedalong the Z direction, as shown in FIG. 11. The expansion of the presenttarget 1110′ as it travels from the first target location 1122A to thesecond target location 1122B in the X direction and the acquired speedin the Z direction depend on an energy of the preliminary radiationpulse 1135A, and in particular, on the energy delivered to (that is,intercepted by) the present target 1110′.

The optical source 140 can be designed to produce a beam of preliminaryradiation pulses 1135A and a beam of main radiation pulses 1135Bdirected to respective target locations 1122A, 1122B. Moreover, asdiscussed above, the EUV light source 100 adjusts one or morecharacteristics of the radiation pulse 135 that is directed to thetarget space 120 based on the analysis of the determined moving propertyor properties of the current target 110. Accordingly, it is possible forthe EUV light source 100 to adjust one or more characteristics of thepreliminary radiation pulse 1135A, one or more characteristics of themain radiation pulse 1135B, or one or more characteristics of both ofthe preliminary radiation pulse 1135A and the main radiation pulse1135B.

Referring to FIG. 12, an exemplary optical source 1240 is designed toproduce the beam of preliminary radiation pulses 1135A and the beam ofmain radiation pulses 1135B directed toward their respective targetlocations 1122A, 1122B within the target space 1120.

The optical source 1240 includes a first optical amplifier system 1200that includes a series of one or more optical amplifiers through whichthe beam of preliminary radiation pulses 1135A is passed, and a secondoptical amplifier system 1205 that includes a series of one or moreoptical amplifiers through which the beam of main radiation pulses 1135Bis passed. One or more amplifiers from the first system 1200 can be inthe second system 1205; or one or more amplifiers in the second system1205 can be in the first system 1200. Alternatively, it is possible thatthe first optical amplifier system 1200 is entirely separate from thesecond optical amplifier system 1205.

Additionally, though not required, the optical source 1240 can include afirst light generator 1210 that produces a first pulsed light beam 1211and a second light generator 1215 that produces a second pulsed lightbeam 1216. The light generators 1210, 1215 can each be, for example, alaser, a seed laser such as a master oscillator, or a lamp. An exemplarylight generator that can be used as the light generator 1210, 1215 is aQ-switched, radio frequency (RF) pumped, axial flow, carbon dioxide(CO₂) oscillator that can operate at a repetition rate of, for example,100 kHz.

The optical amplifiers within the optical amplifier systems 1200, 1205each contain a gain medium on a respective beam path, along which alight beam 1211, 1216 from the respective light generator 1210, 1215propagates. When the gain medium of the optical amplifier is excited,the gain medium provides photons to the light beam, amplifying the lightbeam 1211, 1216 to produce the amplified light beam that forms thepreliminary radiation pulse beam 1135A or the main radiation pulse beam1135B.

The wavelengths of the light beams 1211, 1216 or the radiation pulsebeams 1135A, 1135B can be distinct from each other so that the radiationpulse beams 1135A, 1135B can be separated from each other, if they arecombined at any point within the optical source 1240. If the radiationpulse beams 1135A, 1135B are produced by CO₂ amplifiers, then thepreliminary radiation pulse beam 1135A can have a wavelength of 10.26micrometers (μm) or 10.207 μm, and the main radiation pulse beam 1135Bcan have a wavelength of 10.59 μm. The wavelengths are chosen to moreeasily enable separation of the beams 1135A, 1135B using dispersiveoptics or dichroic mirror or beamsplitter coatings. In the situation inwhich both beams 1135A, 1135B propagate together in the same amplifierchain (for example, a situation in which some of the amplifiers ofoptical amplifier system 1200 are in the optical amplifier system 1205),then the distinct wavelengths can be used to adjust a relative gainbetween the two beams 1135A, 1135B even though they are traversingthrough the same amplifiers.

For example, the beams 1135A, 1135B, once separated, could be steered orfocused to two separate locations (such as the first and second targetlocations 1122A, 1122B, respectively) within the chamber 175. Inparticular, the separation of the beams 1135A, 1135B also enables thetarget 1110 to expand after interacting with the beam of preliminaryradiation pulses 1135A while it travels from the first target location1122A to the second target location 1122B.

The optical source 1240 can include a beam path combiner 1225 thatoverlays the beam of preliminary radiation pulses 1135A and the beam ofmain radiation pulses 1135B and places the beams 1135A, 1135B on thesame optical path for at least some of the distance between the opticalsource 1240 and the beam delivery system 185. Additionally, the opticalsource 1240 can include a beam path separator 1226 that separates thebeam of preliminary radiation pulses 1135A from the beam of mainradiation pulses 1135B so that the two beams 1135A, 1135B can beseparately steered and focused within the chamber 175.

Additionally, the beam of preliminary radiation pulses 1135A can beconfigured to have less pulse energy than the pulse energy of the beamof main radiation pulses 1135B. This is because the preliminaryradiation pulse 1135A is used to modify the geometry of the presenttarget 1110′ while the main radiation pulse 1135B is used to convert themodified present target 1110′ into plasma 1130. For example, the energyof the preliminary radiation pulse 1135A can be 5-100 times less thanthe energy of the main radiation pulse 1135B.

In some implementations, each optical amplifier system 1200, 1205includes a set of three optical amplifiers, though as few as oneamplifier or more than three amplifiers can be used. In someimplementations, each of the optical amplifiers in each system 1200,1205 includes a gain medium that includes CO₂ and can amplify light at awavelength of between about 9.1 μm and about 11.0 μm, and in particular,at about 10.6 μm, at a gain greater than 1000. It is possible for theoptical amplifiers in each system 1200, 1205 to be operated similarly orat different wavelengths. Suitable amplifiers and lasers for use in theoptical amplifier systems 1200, 1205 can include a pulsed laser devicesuch as a pulsed gas-discharge CO₂ amplifier producing radiation atabout 9.3 μm or about 10.6 μm, for example, with DC or RF excitation,operating at relatively high power, for example, 10 kW or higher andhigh pulse repetition rate, for example, 50 kHz or more. Exemplaryoptical amplifiers that can be used in each of the systems 1200, 1205are axial flow high-power CO₂ lasers with wear-free gas circulation andcapacitive RF excitation.

Additionally, though not required, one or more of the optical amplifiersystems 1200 and 1205 can include a first amplifier that acts as apre-amplifier. The pre-amplifier, if present, can be a diffusion-cooledCO₂ laser system.

The optical amplifier systems 1200, 1205 can include optical elementsthat are not shown in FIG. 12 for directing and shaping the respectivelight beams 1211, 1216. For example, the optical amplifier systems 1200,1205 can include reflective optics such as mirrors,partially-transmissive optics such as beam splitters orpartially-transmissive mirrors, and dichroic beam splitters.

The optical source 1240 also includes an optical system 1220 that caninclude one or more optics (such as reflective optics such as mirrors,partially reflective and partially transmissive optics such asbeamsplitters, refractive optics such as prisms or lenses, passiveoptics, active optics, etc.) for directing the light beams 1211, 1216through the optical source 1240.

Although the optical amplifiers can be separate and dedicated systems,it is possible for at least one of the amplifiers of the opticalamplifier system 1200 to be in the optical amplifier system 1205 and forat least one of the amplifiers of the optical amplifier system 1205 tobe in the optical amplifier system 1200. In such a system in which atleast some of the amplifiers and optics overlap between the opticalamplifier systems 1200, 1205, it is possible that the beam ofpreliminary radiation pulses 1135A and the beam of main radiation pulses1135B are coupled together such that changes of one or morecharacteristics of the beam 1135A can cause changes to one or morecharacteristics of the beam 1135B, and vice versa.

Referring to FIG. 13, a procedure 1300 is performed by the EUV lightsource 100 (under control of the control system 170, 470, 670 or 870)for compensating for plasma pushback forces 125 on a present target110′. Other procedures not discussed herein can be performed by the EUVlight source 100 during operation. The procedure 1300 includes formingthe remaining plasma 130 that at least partially coincides with theextended target region 115, the remaining plasma being a plasma formedfrom an interaction between a prior target 110P and a prior radiationpulse 135P in a target space 120 (1305). As shown in FIGS. 14A and 14B,the prior target 110P is approaching the target location 122 as theprior radiation pulse 135P is approaching the target location 122. Afterthe prior radiation pulse 135P and the prior target 110P haveinteracted, the remaining plasma 130 is formed and plasma pushbackforces 125 are produced, as shown in FIGS. 15A and 15B.

The current target 110 is released from the target delivery system 145along the trajectory TR toward the target space 120 (1310). The currenttarget 110 can be released (1310) prior to the remaining plasma 130being formed from the interaction between the prior target 110P and theprior radiation pulse 135P (1305). For example, as shown in FIGS. 14Aand 14B, the current target 110 has been released from the targetdelivery system 145 along the trajectory TR toward the target space 120(1310).

One or more moving properties of the current target 110 (when thecurrent target 110 is within the extended target region 115) aredetermined (1315). The moving property of the current target can bedetermined (1315) by detecting a first interaction between a firstdiagnostic light beam (such as beam 320) and the current target 110 at afirst location (such as location 322) within the extended target region115, detecting a second interaction between a second diagnostic lightbeam (such as beam 330) and the current target 110 at a second location(such as location 328) within the extended target region 115. The firstdiagnostic light beam (such as beam 320) is directed toward the currenttarget 110 at the first location (such as location 322), and the seconddiagnostic light beam (such as beam 330) is directed toward the currenttarget 110 at the second location (such as location 328).

The first interaction can be detected (for example, at the detectionmodule 335) by detecting at least a portion of the first diagnosticlight beam (such as light beam 320) that is reflected from the currenttarget (for example, the light 340 is detected). The second interactioncan be detected (for example, at the detection module 335) by detectinga portion of the second diagnostic light beam (such as light beam 33)that is reflected from the current target 110 (for example, the light350 is detected) by the detection module 335. The moving property orproperties of the current target 110 can be determined (1315) based onthese detections of the reflected portions.

For example, with reference to FIGS. 16A-17B, the diagnostic system 305is used in combination with the control system 170, 470, 670, 870 todetermine the one or more moving properties of the current target 110.In FIGS. 16A and 16B, the current target 110 interacts with thediagnostic light beam 320, and the light 340 from that interaction isdetected by the detection module 335. In FIGS. 17A and 17B, the currenttarget 110 then interacts with the diagnostic light beam 330, and thelight 350 from that interaction is detected by the detection module 335.The detection module 335 outputs the data to the control system 170,470, 670, 870 for processing, as discussed above, to determine the oneor more moving properties of the current target 110.

The control system 170, 470, 670, 870 determines whether any of thedetermined moving properties are outside of an acceptable range (1320).If any of the moving properties is out of an acceptable range (1320),then the control system 170, 470, 670, 870 adjusts one or morecharacteristics of the radiation pulse 135 (for example, one or morecharacteristics of one or more of the preliminary radiation pulse 1135Aand the main radiation pulse 1135B) to thereby control a relativeposition between the radiation pulse 135 and the present target 110′based on the determined moving property or properties of the currenttarget 110 (1325). The radiation pulse 135 (which may have been adjustedat 1325) is directed toward the target space 120 so that the radiationpulse 135 and the present target 110′ interact in the target space 120(1330). For example, as shown in FIGS. 18A and 18B, the present target110′ is approaching the target location 122 within the target space 120and the adjustments have been made to the radiation pulse 135, which isalso directed toward the target location. And, as shown in FIGS. 19A and19B, the present target 110′ is interacting with the current radiationpulse 135 at the target location 122.

The moving property or properties that can be determined (1315) includeone or more of a speed, velocity, direction, acceleration, or locationof the current target 110 along any of the directions X, Y, or Z of thethree dimensional coordinate system.

In some implementations, such as shown in FIG. 11, the radiation pulse135 can be a preliminary radiation pulse 1135A that delivers the energyto the present target 110′ to modify a geometric distribution of thepresent target 110′. If this occurs, then the procedure 1300 can alsoinclude, after directing the current preliminary radiation pulse 1135Atoward the present target 110′, directing a main radiation pulse 1135Btoward the present target 110′ to thereby convert at least part of thepresent target 110′ into plasma that emits EUV light 1150. FIGS. 19C and19D show the interaction between the main radiation pulse 1135B and thepresent target 110′ to produce the EUV light 1150.

The procedure 1300 can also include analyzing the one or more movingproperties that were determined (1315). For example, the control system170, 470, 670, 870 can determine the velocity of the current target 110along the −X direction and predict when the present target 110′ willreach the target location 122. The control system 170, 470, 670, 870 canadjust when the radiation pulse 135 is released or it can adjust thedirection of the radiation pulse 135 so that the radiation pulse 135 andthe present target 110′ efficiently interact at the target location 122(1325). This adjustment to the relative position between the radiationpulse 135 and the present target 110′ is therefore based on the analysisof the determined moving property of the current target 110.

As also shown in FIG. 19C, the next current target 110N is released at apoint in time in accordance with the rate at which the targets 110 arereleased from the target delivery system 145.

In some implementations, the acceleration A of the current target 110can be determined (1315) as well as the velocity V. In suchimplementation, the determination (1315) would additionally includedetecting a third interaction between a third diagnostic light beam andthe current target at a third location within the extended targetregion, the third location being distinct from the first and secondlocations. For example, as shown in FIGS. 20A and 20B, the currenttarget 110 is directed toward the target space 120, and, while in theextended target region 115, the current target 110 would interactsequentially with the diagnostic light beams 720, 725, 730 at respectivelocations 722, 724, 728. As discussed above, the resultant light 740,745, 750 is detected by the detection module 735, which outputs datathat is analyzed by the control system 170, 470, 670, 870, which can usethe data to determine the acceleration A as well as the velocity V ofthe current target 110. Additionally, the control system 170, 470, 670,870 can use the additional information obtained from the interactionbetween the current target 110 and the third diagnostic beam 725 todetermine one or more moving properties of the current target 110 alonga direction (such as the Y direction) perpendicular to the −X direction.

Referring again to FIG. 3, in other implementations, the detectionmodule 335 of the diagnostic system 305 is designed to detect,additionally or alternatively, a two-dimensional representation (such asan image) of the light 340 and 350 produced from the interaction betweenrespective diagnostic light beam 320 and 330 and the target 110 at thedistinct diagnostic locations 322 and 328. To this end, the detectionmodule 335 includes an image recording device (such as a camera), asdiscussed below. Moreover, it is possible to implement the detection oftwo-dimensional representations of the light 540, 550 and 740, 745, 750in the respective detection modules 535 and 735 as well. In someimplementations, the control system 170, 470, 670, 870 can analyze thetwo-dimensional representation to determine all of the moving propertiesof the target 110 without the need for analyzing the one-dimensionalaspect (such as the intensity of the light).

Alternatively, it is possible to configure a diagnostic system 105 todetect and record two-dimensional representations and use only theinformation from these two-dimensional representations to determine oneor more moving properties of the target 110. Initially, designs that useonly the two-dimensional representations are described and discussedbelow with reference to FIGS. 24 to 26, followed by a description anddiscussion of designs that use both the two-dimensional representationsas well as the one-dimensional aspect (such as the intensity of thelight) with reference to FIG. 27.

An exemplary design of a diagnostic system 2105 that is configured todetect a two-dimensional representation (such as an image) of the lightproduced from the interaction between a diagnostic probe (for example, alight beam) and the target 110 is shown in FIG. 21A, and is describednext. By detecting a two-dimensional representation of the light, thediagnostic system 2105 provides enough information to the control system170 to enable all of the diagnostic capabilities, as discussed next. Thediagnostic system 2105 is configured to detect the two-dimensionalrepresentation of the light produced from the interactions between thediagnostic probe and each target 110 that is emitted from the targetdelivery system 145.

For example, the control system 170 can determine a position of thecurrent target 110 along any of the X, Y, and Z directions of the threedimensional coordinate system within close range (for example, 1 mm) ofa focus of the current radiation pulse 135. The control system 170 candetermine a velocity of the current target 110 along any of the X, Y,and Z directions, and at the same time adjust one or morecharacteristics of the radiation pulse 135 that interacts with thepresent target 110′ prior to another target 110 entering the extendedtarget region 115. This detection and analysis can be performed forevery target 110 and if the actuation system for modifying the radiationpulse 135 is fast enough, the feedback control can be performed on thesame target 110 for which detection and analysis is performed. Thecontrol system 170 can determine the moving properties (such as theposition) of the current target 110 along any of the X, Y, or Zdirections. The control system 170 can determine the moving property orproperties of each target 110 along any or all of the X, Y, and Zdirections, and adjust one or more characteristics of the radiationpulse 135 that interacts with the present target 110′ for each target110 that enters the extended target region 115. Thus, this determinationand adjustment is made at a repetition rate that is greater than orequal to the rate at which targets 110 are released from the targetdelivery system 145. For example, this repetition rate can be at least50 KHz. This means that the determination of the moving properties ofthe current target 110 and the adjustment of the radiation pulse 135that interacts with the present target 110′ occurs within a 20 μs timeframe. The control system 170 can determine dynamics and motion of thetarget 110 near the focus of the radiation pulse 135 to observe effectsof the plasma pushback force 125 on the target 110.

The diagnostic system 2105 includes a probe module 2100 that is designedto produce at least two diagnostic probes 2120, 2130. While only twodiagnostic probes 2120, 2130 are shown in FIG. 21A, it is possible formore than two to be used. A diagnostic probe 2120 or 2130 can interactwith the target 110 at one or more locations along its trajectory TR andat one or more times. For example, the diagnostic probe 2120 caninteract with the target 110 at a first location L_(TR1) and a firsttime T₂₁₂₀, and the diagnostic probe 2130 can interact with the target110 at a second location L_(TR2) and a second time T₂₁₃₀. As anotherexample, the diagnostic probe 2120 can interact with the target 110 atboth the first time T₂₁₂₀ and the second time T₂₁₃₀, and the diagnosticprobe 2130 can interact with the target 110 at both the first time T₂₁₂₀and the second time T₂₁₃₀.

The probe module 2100 can be an imaging module that produces diagnosticlight beams as the diagnostic probes 2120, 2130. In someimplementations, the probe module 2100 is a laser source that produces,as the diagnostic probes 2120, 2130, diagnostic laser beams. Thewavelength of the diagnostic probes 2120, 2130 impacts or affects otheraspects of the interaction with the target 110. For example, thewavelength of the diagnostic probes 2120, 2130 can impact whether thetarget 110 is scattered. As another example, any one or more of theresponsivity, the optical resolution, the sampling rate, the frame rate,and the exposure time of the detection module 2135 (discussed below)depends on the wavelength of the diagnostic probes 2120, 2130.

The diagnostic laser beams can have a wavelength that is distinct fromthe wavelength of the radiation pulse 135 and can have a power that islow enough to prevent or reduce any interference between the diagnosticlight beam 2120, 2130 and the target 110. The wavelength of thediagnostic probes 2120, 2130 is also selected to avoid or reduce overlapwith the emission lines of the plasma 130 produced in the target space120. The wavelength of the diagnostic probes 2120, 2130 should beselected to match with or coincide with the wavelength bands of any ofthe optical elements through which the diagnostic probes 2120, 2130 aredirected. Moreover other aspects, such as the beam quality, stability,power level, of the diagnostic probes 2120, 2130 can be selected oradjusted depending on the application. The wavelength of the diagnosticprobes 2120, 2130 is selected to enable a high signal to noise ratiothat is recorded by the detection module 2135 (discussed below). Forexample, the wavelength of the diagnostic light beam 2120, 2130 can bein the infrared region, such as about 1030 nm, and can have a power ofabout 9.6 W. The diagnostic light beams 2120, 2130 can be laser light.In other examples, the wavelength of the diagnostic light beam 2120,2130 is in the visible region.

The diagnostic light beams 2120, 2130 can be collimated beams of light(as shown in FIGS. 24 and 25) or can be beams of light that are focusedon or near the trajectory TR (as shown in FIGS. 26 and 27). Thediagnostic light beams 2120, 2130 can be directed along a direction thatis in a plane defined by the X and Y directions (the XY plane). As shownin the example of FIG. 21A, the diagnostic light beams 2120, 2130 aredirected along the −Y direction. The diagnostic light beams 2120, 2130can overlap with each other depending on their directions. Moreover, itis possible for the diagnostic light beams 2120, 2130 to overlap witheach other (as shown in FIG. 24) or to overlap with the target 110 at aplurality of locations and times as it travels along a distance alongthe trajectory TR, as shown in FIGS. 24 and 25.

The probe module 2100 can be an adjustable continuous wave laserproducing diagnostic light beams 2120, 2130 at an infrared wavelengthfor example, between 1020 nm and 1070 nm, and a power of up to 50W, witha beam quality M² approaching 1. The laser can be a fiber laser source.In other implementations, the probe module 2100 can be a pulsed laser.

In some implementations, the diagnostic light beams 2120, 2130 can becontinuously produced as a curtain that crosses the trajectory TR. Inother implementations, the diagnostic light beams 2120, 2130 areproduced only at certain times, for example, only when the target 110 isexpected to be at a specific location along the trajectory TR. In thiscase, the probe module 2100 can be triggered (or pulsed) by a timingsignal from the control system to produce one or more diagnostic lightbeams 2120, 2130 at specific times.

The diagnostic system 2105 also includes a detection module 2135 thatrecords one or more two-dimensional representations 2141, 2151 of light2140, 2150 that is produced due to the interaction between the currenttarget 110 and one or more of the diagnostic probes 2120, 2130 as thetarget 110 travels along the trajectory TR through the extended targetregion 115. The detection module 2135 includes one or moretwo-dimensional recording devices 2135A, 2135B for recording thetwo-dimensional representations 2141, 2151. The detection module 2135can also include other optical elements such as imaging lenses ormirrors, as needed, as discussed below.

Although only two recording devices 2135A, 2135B are shown in FIG. 21A,more than two or only one recording device can be used, depending on theapplication. Specifically, at least two recording devices 2135A, 2135Bare used in order to gather enough information about the motion of thetarget 110 to reconstruct the trajectory of the target 110 in all threedirections (X, Y, and Z). If reasonable assumptions regarding theacceleration of the target 110 are made, then the trajectory of thetarget 110 in all three directions (X, Y, and Z) can be reconstructedwith just one recording device.

In some implementations in which the diagnostic probes 2120, 2130 arelight beams, the light 2140, 2150 that is produced is the diagnosticlight beam 2120, 2130 that traverses or passes across the target 110,such traversed light including a shadow of the target 110 obscuring atleast a portion of the diagnostic light beam, as shown in FIG. 24. Thissort of arrangement produces shadows within the traversed light beam asthe two-dimensional representations 2141, 2151 and can be considered ashadowgraph arrangement. In such an implementation, the two-dimensionalrecording device 2135A, 2135B can be arranged on a side of the targettrajectory TR that is opposite to the side on which the probe module2100 is arranged.

In other implementations in which the diagnostic probes 2120, 2130 arelight beams, the light 2140, 2150 that is produced is the light that isscattered or reflected from the current target 110 as it travels alongits trajectory TR, as shown in FIGS. 25-27. In such an implementation,the two-dimensional recording device 2135A, 2135B can be arranged on aside of the target trajectory TR that is the same side on which theprobe module 2100 is arranged.

Each two-dimensional recording device 2135A, 2135B can be a camera thatcaptures the two-dimensional representation 2141, 2151 (which can beconsidered an image) of the light 2140, 2150. Thus, for example, thetwo-dimensional recording device 2135A, 2135B includes a two-dimensionalarray of thousands or millions of photo-sites (or pixels). The light2140, 2150 is directed onto the photo-sensitive area of each pixel whereit is converted into electrons that are collected into a voltage signaland the array of these signals forms the two-dimensional image 2141,2151. The recording device 2135A is arranged so that its two-dimensionalarray is in a plane defined by the following two axes: a first axis,which is the Z direction, and a second axis, which is an axis lying inthe XY plane. Thus, the normal to the two-dimensional array of therecording device 2135A is in the XY plane. As shown in the example ofFIG. 21A, the recording device 2135A is arranged so that itstwo-dimensional array is in the XZ plane and its normal is along the Ydirection. Similarly, the recording device 2135B is arranged so that itstwo-dimensional array is in a plane defined by a first axis which is theZ direction and a second axis, which is an axis in the XY plane. Thus,the normal to the two-dimensional array of the recording device 2135B isin the XY plane. As shown in the example of FIG. 21A, the recordingdevice 2135B is also arranged so that its two-dimensional array is inthe XZ plane and its normal is along the Y direction. Other exemplaryarrangements are shown below.

The two-dimensional recording device 2135A, 2135B can be controlled bythe control system 170 to record an image at a specific time. Thetwo-dimensional recording device 2135A, 2135B sends the representations2141, 2151 to the control system 170 for analysis. The control system170 analyzes the two-dimensional representations 2141, 2151 to determinemoving properties of the target 110 along one or more of the X, Y, and Zdirections.

The recording devices 2135A, 2135B should be “high speed” cameras thatare fast enough to detect, record, and output the two-dimensional image2141, 2151 of the light 2140, 2150 for a current target 110 before thenext target enters the extended target region 115 but after the priortarget 110P has interacted with the prior radiation pulse 135P. Theframe rate of the camera should be greater than or equal to the rate atwhich the target delivery system 145 generates targets to enable thediagnostic system 2105 to perform an analysis on every target directedtoward the target space 120. Thus, if the target delivery rate is 50kHz, then the frame rate of the camera should be greater than or equalto 50 kHz. An example of a suitable high speed camera 2135A, 2135B is acomplementary metal-oxide semiconductor (CMOS). The cameras can have anexposure time of about 300 μs, an exemplary resolution of about1696×1710 pixels, a pixel size of about 8 μm, and a gain of 1.0.

In other implementations, the camera 2135A, 2135B is a charged coupleddevice (CCD) or infrared camera.

As discussed herein, the diagnostic system 2105 is useful fordetermining the moving properties of the target 110 in the extendedtarget region 115. Nevertheless, the diagnostic system 2105 can beuseful for determining the moving properties of the target 110 atlocations other than those within the extended target region 115. Thus,in other implementations, the diagnostic system 2105 is set up in aregion outside of the extended target region 115. For example, in suchan implementation, the diagnostic system 2105 is set up so that thediagnostic probes 2120, 2130 interact with the target 110 eithercompletely or partially in the first region 165, which is between theextended target region 115 and the target delivery system 145. The firstregion 165 can be considered as a region in which the plasma pushbackforces 125 have a much lower or insignificant effect on the currenttarget 110.

As discussed above, the target 110 interacts with one or more of thediagnostic probes 2120, 2130 at two locations, namely, the firstlocation L_(TR1) at the first time T₂₁₂₀ and the second location L_(TR2)at the second time T₂₁₃₀. By performing an interaction between thetarget 110 and one or more diagnostic probes 2120, 2130 at twolocations, the control system 170 can derive the position P1(X,Y,Z) ofthe target 110 in the X, Y, and Z directions of the chamber 175 at thelocation L_(TR1) and the position P2(X,Y,Z) of the target 110 in the X,Y, and Z directions of the chamber 175 at the location L_(TR2).Specifically, the positions P1(X,Y,Z) and P2(X,Y,Z) of the target 110within the chamber 175 can be determined by first identifying thelocation of one or more regions of interest (ROI) within thetwo-dimensional images 2141, 2151 captured by the sensors of therecording devices 2135A, 2135B. The positions P1(X,Y,Z) and P2(X,Y,Z) ofthe target 110 are calculated using one or more of the following data:the one or more regions of interest (ROIs) in the recorded images, thetime that the target 110 crosses the first location L_(TR1) (whichcorresponds to the time that the target 110 interacts with thediagnostic probe 2120), the time that the target 110 crosses the secondlocation L_(TR2) (which corresponds to the time that the target 110interacts with the diagnostic probe 2130), and the respective planes ofthe sensors of the recording devices 2135A, 2135B. The plane of thesensor 2135A is defined by two lines: a line along the Zs direction,which is parallel with the Z direction of the chamber 175, and a linealong a direction in the Xs-Ys plane, in which the Xs direction isparallel with the X direction of the chamber 175 and the Ys direction isparallel with the Y direction of the chamber 175.

With reference to FIG. 21B, the one or more regions of interest (ROIs)within an image correspond to the pixels in the image at which the light2140, 2150 strikes the sensor of the respective recording devices 2135A,2135B. For example, with reference to FIG. 21B, an exemplaryrepresentation 2151 captured by the camera 2135B is shown. In thisexample, the control system 170 identifies a single ROI that representsthe area within the image that corresponds to the location of the target110 as it passes across the location L_(TR2). The control system 170analyzes this representation 2151 and determines the center moment ofthe ROI which can be referred to as the centroid. The center moment ofthe ROI can be designated as C(Xs,Ys,Zs).

For example, the centroid of the ROI along the Y direction could begiven by this calculation:

${{{Centroid}\mspace{11mu} (Y)} = \frac{\sum{{mi}\; {Yi}}}{\sum{mi}}},$

where mi is the value of the voltage or current at pixel i and Yi is thecoordinate of pixel i and I is contained within the ROI. A similarcalculation can be performed for the X and Z directions. Moreover, asimilar analysis is performed for the representation 2141 captured bythe camera 2135A.

Once the center moment of the ROI is set, the control system 170 definesthe complete ROI be adjusting the number of pixel rows and columns thatare read out, the number of pixels that are read out depending on theframe rate of the recording device 2135A, 2135B. The ROI can be definedto match the volume of the target 110 that is being viewed.

Once the control system 170 identifies each ROI within therepresentation 2151, the control system 170 analyzes the ROI todetermine the position P2 of the target 110 along the X, Y, and Zdirections at the location L_(TR2) within the chamber 175. For example,the control system 170 could determine a center moment (such as acentroid) C(Xs,Ys,Zs) of each ROI along each of the Xs, Ys, and Zsdirections, and once each centroid C(Xs,Ys,Zs) is determined, thecontrol system 170 can estimate the position P2 of the target 110 alongthe X, Y, and Z directions at the location L_(TR2) of the chamber 175.

The position P2 of the target 110 along the Z direction of the chamber175 is linearly correlated with the center of the ROI (the centroid)along the Zs direction (CZs) of the sensor because the Zs direction ofthe sensor of the recording device 2135A is parallel with the Zdirection of the chamber 175. Thus, P2(Z) =Factor*CZs, where the Factoris a constant value that can depend on one or more of a size of a pixeland an optical magnification of the recording device 2135B or 2135A.

The position P2 of the target 110 along the X direction of the chamber175 and the position P2 of the target 110 along the Y direction of thechamber 175 can be estimated using the centroid taken along the Xs-Ysdirection (CXsYs). However, because the centroid along the Xs-Ysdirection CXsYs lies in the Xs-Ys plane, it is not possible to determinethe position P2 of the target 110 along the X direction of the chamber175 and the position P2 of the target 110 along the Y direction of thechamber 175 without additional information.

In some implementations, the additional information that is used fordetermining the position P2 along both the X direction and the Ydirection of the chamber 175 includes obtaining an image of the target110 at the location L_(TR2) at a second recording device (such asrecording device 2135A) that has a sensor that is in a plane that isdistinct from the plane of the sensor of the recording device 2135B. Inother implementations the additional information that is used could be atime difference between the time T₂₁₂₀ that the target 110 crosses thelocation L_(TR1) and the time T₂₁₃₀ that the target 110 crosses thelocation L_(TR2).

Other additional information that is needed to transform the values ofthe centroid at the sensor plane of the recording device 2135B (or2135A) to the coordinate system of the chamber 175 includes the angle atwhich the recording device 2135B (and 2135A) is positioned relative tothe X and Y directions of the chamber 175. This value is a known valueand can be measured or determined when the recording devices 2135A,2135B are set up in the chamber 175.

Once the positions P1(X,Y,Z) and P2(X,Y,Z) are determined, the controlsystem 170 can derive the average velocity V(X,Y,Z) of the target 110 inthe X, Y, and Z directions between the locations L_(TR1) and L_(TR2), asfollows. The average velocity V(X) in the X direction is given by[P2(X)−P1(X)]/[T2130−T2120]; the average velocity V(Y) in the Ydirection is given by [P2(Y)−P1(Y)]/[T2130−T2120]; and the averagevelocity V(Z) in the Z direction is given by[P2(Z)−P1(Z)]/[T2130−T2120].

In another exemplary implementation, the target 110 interacts with oneor more of the diagnostic probes 2120, 2130 (or a third diagnostic probenot shown) at a third location L_(TR3) that is distinct from the firstlocation L_(TR1) and the second location L_(TR2). For example, the thirdlocation L_(TR3) could be between the second location L_(TR2) and thetarget space 120 along the trajectory TR. This additional interaction isrecorded by the detection module 2135. The control system 170 can deriveadditional information about the moving properties of the target 110.For example, the control system 170 can derive the position P3(X,Y,Z) ofthe target 110 in the X, Y, and Z directions at the third locationL_(TR3). The control system 170 can also derive the accelerationA(X,Y,Z) of the target 110 along each of the X, Y, and Z directions,using the following three linear equations.

The first equation provides the relationship between the position of thetarget 110 along the X direction at the location L_(TR3), P3(X), and theacceleration A23(X) of the target 110 along the X direction between thelocation L_(TR2) and the location L_(TR3) as follows:

P3(X)=P2(X)+V(X)*T₂₃+½A23(X)*T₂₃ ², where T₂₃ is the time it takes thetarget 110 to travel from the location L_(TR2) to the location L_(TR3).

The second equation provides the relationship between the position ofthe target 110 along the Y direction at the location L_(TR3), P3(Y), andthe acceleration A23(Y) of the target 110 along the Y direction asfollows:

P3(Y)=P2(Y)+V(Y)*T₂₃+½A23(Y)*T₂₃ ², where T₂₃ is the time it takes thetarget 110 to travel from the location L_(TR2) to the location L_(TR3).

The third equation provides the relationship between the position of thetarget 110 along the Z direction at the location L_(TR3), P3(Z), and theacceleration A23(Z) of the target 110 along the Z direction as follows:

P3(Z)=P2(Z)+V(Z)*T₂₃+½A23(Z)*T₂₃ ², where T₂₃ is the time it takes thetarget 110 to travel from the location L_(TR2) to the location L_(TR3).

These three linear equations can be solved for A23(X), A23(Y), andA23(Z). This exemplary approach that uses linear equations can beextended to any number of locations L and positions P(X,Y,Z) to obtainadditional data for determining the trajectory of the target 110.

Referring to FIG. 22, an exemplary control system 2270 is shown. Thecontrol system 2270 is configured to control the diagnostic system 2105,analyze information from the diagnostic system 2105, and determine howto modify aspects of the light source 100 based on this analysis. Tothis end, the control system 2270 includes a diagnostic sub-controller2200 that includes a trigger source 2271, an analysis module 2272, and adecision module 2274. The output of the decision module 2274 is sent toone or more of the optical source sub-controller 2205, the beam deliverysub-controller 2210, the target delivery sub-controller 2215, and othersub-controllers 2220 that can control other aspects of the light source100. The output of the decision module 2274 can also be sent to a probesub-controller 2212, which controls operation of the probe module 2100.

The trigger source 2271 can be any suitable source that provides one ormore digital trigger signals to the diagnostic system 2105 in order toinstruct one or more components of the diagnostic system 2105 tooperate. In some implementations, the trigger source 2271 is associatedwith the release of the target 110 from the target delivery system 145.In other implementations, the trigger source 2271 is associated with anoutput from a photon detection device such as a photodiode placed alongthe trajectory TR to detect light scattered from the target 110 at aspecific location. In such an implementation, the trigger source 2271could include a discriminator that receives the output from thephotodiode and outputs one or more digital time stamp signals. Thetrigger signal or signals from the trigger source 2271 are supplied tothe detection module 2135 in order to instruct the one or moretwo-dimensional recording devices 2135A, 2135B when to record thetwo-dimensional representations. It is also possible for the triggersource 2271 to include a time delay signal that is added to each triggersignal before outputting to the detection module 2135, depending on theinitiation of the trigger signal. For example, if the trigger source2271 receives the output from a photodiode for a prior target 110P, thena time delay signal can be added to each trigger signal in order tooperate the detection module 2135 to record light only when the currenttarget 110 passes the diagnostic probes 2120, 2130.

Moreover, each trigger signal from the trigger source 2271 can have alength or duration that is variable or adjustable, depending on how longthe recording devices 2135A, 2135B in the detection module 2135 shouldbe recording the light produced from the interaction between the target110 and the diagnostic probes 2120, 2130. Thus, the duration of eachtrigger signal from the trigger source 2271 acts as a shutter on therecording devices 2135A, 2135B.

Referring to FIG. 22, the analysis module 2272 receives thetwo-dimensional representations (the images) from the detection module2135, and performs processing on the images. The analysis module 2272includes various sub-modules that are configured to perform varioustypes of analysis on the images. For example, the analysis module 2272can include an input sub-module 2300 that receives the images from thedetection module 2135 and converts the data into a format suitable forprocessing. The analysis module 2272 can include a pre-processingsub-module 2305 that prepares the images from the detection module 2135(for example, removing background noise, filtering the images, and gaincompensation). The analysis module 2272 can include an image sub-module2310 that processes the image data such as identifying one or moreregions of interest (ROIs) within an image, where each ROI correspondsto a location of the target 110 along its trajectory TR. The imagesub-module 2310 also calculates an area of each ROI in the image andcalculates a centroid of each region of interest. The analysis module2272 can include an output sub-module 2315 that prepares the calculateddata (such as the area and centroid of the ROIs) for output to thedecision module 2274.

The decision module 2274 determines the one or more moving properties ofthe target 110 based on the output from the analysis module 2272, anddetermines whether any of the moving properties are outside anacceptable range. The decision module 2274 also determines whether otheraspects of the light source 100 need to be adjusted if any of the movingproperties are outside of an acceptable range.

Referring to FIG. 24, an exemplary diagnostic system 2405 is designed ina shadowgraph arrangement in which light illuminates the target 110 fromone side of the target 110 while the imaging is performed at the otherside of the target 110. The diagnostic system 2405 includes a probemodule 2400 and a detection module 2435.

The probe module 2400 includes two or more light sources 2402, 2404,each light source 2402, 2404 configured to produce and direct arespective diagnostic light beam 2420, 2430 toward the trajectory TR.The diagnostic light beam 2420, 2430 in this example is expanded along atransverse direction to the respective axis A2 and A4 of the light beam2420, 2430 and is collimated by use of a respective refractive optic2412, 2414. The light sources 2402, 2404 can be continuous wave lasersources. In this example, each diagnostic light beam 2420, 2430interacts with the target 110 as it travels along a probe distance D_(p)of its trajectory TR. The probe distance D_(p) has an extent thatenables the detection module 2435 to record light produced from theinteraction between the diagnostic light beam 2420, 2430 and the target110 at a plurality of locations along the trajectory TR. Thus, in theexample of FIG. 24, the interaction between the target 110 and eachdiagnostic light beam 2420, 2430 produces light 2140, 2150,respectively, that is recorded at two times, t1, which corresponds tothe target passing through location L_(TR1), and t2, which correspondsto the target passing through location L_(TR2). Although two recordinglocations are shown in FIG. 24, it is possible to accurately determinemoving properties of the target 110 by recording at only one location orrecording at three or more locations, depending on how much informationabout the moving properties of the target 110 is desired.

The detection module 2435 includes two or more two-dimensional recordingdevices 2435A and 2435B that are arranged on a side of the trajectory TRthat is opposite to a side on which the light sources 2402, 2404 arearranged. In this way, the recording device 2435A is placed to record atwo-dimensional representation 2441 of the light 2440 that is producedfrom the interaction between the diagnostic light beam 2420 and thetarget 110, and the recording device 2435B is placed to record atwo-dimensional representation 2451 of the light 2450 that is producedfrom the interaction between the diagnostic light beam 2430 and thetarget 110. The recording device 2435A is positioned so that the normalto its sensor plane is at an angle αA relative to the X direction of thechamber 175 and the recording device 2435B is positioned so that thenormal to its sensor plane is at an angle αB relative to the X directionof the chamber 175.

There are two ways to arrange the timing of the production of thediagnostic light beam 2420, 2430 and the recording of therepresentations 2441, 2451 at the respective recording devices 2435A,2435B. In both ways, the recording device 2435A records therepresentation 2441 of the light 2440 and the recording device 2435Brecords the representation 2451 of the light 2450.

In a first way, the diagnostic light beam 2420, 2430 is continuouslyproduced and directed to traverse the trajectory TR. The shutters on therecording devices 2435A, 2435B are configured so that therepresentations 2441, 2451 are only recorded at two times for aparticular target; the first time occurs when the target 110 passes thelocation L_(TR1) (at which point the shutters are briefly opened) andthe second time occurs when the target 110 passes the location L_(TR2)(at which point the shutters are briefly opened).

In a second way, the diagnostic light beam 2420, 2430 is pulsed toproduce light at two times; first when the target 110 passes thelocation L_(TR1) and second when the target passes the location L_(TR2).The shutters on the recording devices 2435A, 2435B are open long enoughso that the recording device 2435A records the representation 2441,which includes the light 2440 produced at both times from the pulseddiagnostic beam 2420 as it interacts with the target 110 at locationsL_(TR1) and L_(TR2), and the recording device 2435B records therepresentation 2451, which includes the light 2450 produced at bothtimes from the pulsed the diagnostic beam 2430 as it interacts with thetarget 110 at both locations L_(TR1) and L_(TR2).

The presence of the target 110 as it passes across location L_(TR1)shows up in the representation 2441 as a shadow 2442 formed by thetarget 110 obscuring the diagnostic light beam 2420. The presence of thetarget 110 as it passes across the location L_(TR1) also shows up in therepresentation 2451 as a shadow 2452 formed by the target 110 obscuringthe diagnostic light beam 2430. Similarly, the presence of the target110 as it passes across location L_(TR2) shows up in the representation2441 as a shadow 2443 formed by the target 110 obscuring the diagnosticlight beam 2420. The presence of the target 110 as it passes across thelocation L_(TR2) also shows up in the representation 2451 as a shadow2453 formed by the target 110 obscuring the diagnostic light beam 2430.

Referring to FIG. 25, an exemplary diagnostic system 2505 is designed ina scatter imaging arrangement in which imaging is performed at a side ofthe target 110 that is the same side at which light illuminates thetarget 110. The diagnostic system 2505 includes a probe module 2500 anda detection module 2535.

The probe module 2500 includes a light source 2502 that is configured toproduce and direct a diagnostic light beam 2520 toward the trajectoryTR. The diagnostic light beam 2520 is expanded along its transversedirection to its axis and is collimated by use of a refractive optic2512. The light source 2502 can be a continuous wave laser. Thediagnostic light beam 2520 interacts with the target 110 as it travelsalong a probe distance D_(p) of its trajectory TR. The probe distanceD_(p) has an extent that enables the detection module 2535 to recordlight 2540A, 2540B, 2550A, 2550B produced from the interaction betweenthe diagnostic light beam 2520 and the target 110 at a plurality oflocations L_(TR1) and L_(TR2) that correspond to times t1 and t2,respectively, along the trajectory TR. Although two recording locationsare shown in FIG. 25, it is possible to accurately determine movingproperties of the target 110 by recording at only one location orrecording at three or more locations, depending on how much informationabout the moving properties of the target 110 is desired.

The detection module 2535 includes two or more two-dimensional recordingdevices 2535A and 2535B that are arranged on the same side as the lightsource 2502. In this way, the recording device 2535A is placed to recorda two-dimensional representation 2541 of the light 2540A produced fromthe interaction between the diagnostic light beam 2520 and the target110 at the location L_(TR1) and the light 2540B produced from theinteraction between the diagnostic light beam 2520 and the target 110 atthe location L_(TR2). Additionally, the recording device 2535B is placedto record a two-dimensional representation 2551 of the light 2550Aproduced from the interaction between the diagnostic light beam 2520 andthe target 110 at the location L_(TR1) and the light 2550B produced fromthe interaction between the diagnostic light beam 2520 and the target110 at the location L_(TR2).

Similar to the discussion above, there are two primary ways to time therecording and imaging performed by the diagnostic system 2505. In afirst way, the light beam 2520 is continuously produced and directed totraverse the trajectory TR. The shutters on the recording devices 2535A,2535B are configured so that the representations 2541, 2551 are onlyrecorded at two times for a particular targets; the first time occurs att1 when the target 110 passes the location L_(TR1) (at which point theshutters are briefly opened to acquire data) and the second time occursat t2 when the target 110 passes the location L_(TR2) (at which pointthe shutters are briefly opened again to acquire data). If morelocations are being recorded then the shutters would be configured toopen for those additional locations. In a second way, the light beam2520 is pulsed to produce light at the two times t1 and t2 and theshutters on the recording devices 2535A, 2535B are open long enough tocover the entire probe distance D_(P) and thus capture the interactionsat both pulses.

The presence of the target 110 as it passes across location L_(TR1)shows up in the representation 2551 as a bright spot 2552 formed fromthe light beam 2520 reflecting or scattering off the target 110 towardthe recording device 2535B. The presence of the target 110 as it passesacross location L_(TR1) also shows up in the representation 2541 as abright spot 2542 formed from the light beam 2520 reflecting orscattering off the target 110 toward the recording device 2535A. Thepresence of the target 110 as it passes across location L_(TR2) shows upin the representation 2551 as a bright spot 2553 formed from the lightbeam 2520 reflecting or scattering off the target 110 toward therecording device 2535B. The presence of the target 110 as it passesacross location L_(TR2) also shows up in the representation 2541 as abright spot 2543 formed from the light beam 2520 reflecting orscattering off the target 110 toward the recording device 2535A.

Referring to FIG. 26, in another implementation, the diagnostic system2605 is designed so that the diagnostic light beams 2620, 2625, 2630 arefocused on or near the trajectory TR. The diagnostic system 2605includes a probe module 2600 that includes a single light source 2602that produces a light beam 2610. The diagnostic system 2605 produces thediagnostic light beams 2620, 2625, 2630 that serve as diagnostic probes.

To this end, the probe module 2600 also includes a splitting optic 2615and a refractive optic 2617 such as a focusing lens. The light beam 2610is directed through the splitting optic 2615, which splits the lightbeam 2610 into a plurality of light beams, which travel along distinctdirections and are directed through the refractive optic 2617 to producethe diagnostic light beams 2620, 2625, 2630. The diagnostic light beams2620, 2625, 2630 are directed toward the trajectory TR of the currenttarget 110. The splitting optic 2615 can split the light beam 110 sothat the diagnostic light beams 2620, 2625, 2630 are separated by a setdistance (for example, 0.65 mm) at the trajectory TR. Moreover, therefractive optic 2617 can ensure that the foci (or beam waist) of eachof the diagnostic light beams 2620, 2625, 2630 overlaps the trajectoryTR.

Because of the design of the splitting optic 2615 and the refractiveoptic 2617, the diagnostic light beams 2620, 2625, 2630 are directed sothat they fan out toward the trajectory TR and intersect the trajectoryTR at different and distinct angles. For example, the diagnostic lightbeam 2625 can intersect the trajectory TR at a right or approximatelyright angle to the −X direction. The diagnostic light beam 2620 canintersect the trajectory TR at an angle that is less than 90° relativeto the −X direction and the diagnostic light beam 2630 can intersect thetrajectory TR at an angle that is greater than 90° relative to the −Xdirection. Each of the diagnostic light beams 2620, 2625, 2630 can beGaussian beams so that the transverse profile of the optical intensityof each diagnostic light beam 2620, 2625, 2630 can be described with aGaussian function. The beam waist of each diagnostic light beam 2620,2625, 2630 can be configured to overlap at the trajectory TR or the −Xdirection.

In some implementations, the splitting optic 2615 is a diffractive opticsuch as a rectangular or binary phase diffraction grating that producesdiscrete and spatially spaced replicas of the input light beam 2610. Theseparation between the diagnostic light beams 2620, 2625, 2630 can beadjusted or customized depending on the rate at which the targets arereleased from the target delivery system 145 as well as the size andmaterial of the targets. In some implementations, the splitting optic2615 is a diffractive optic such as a binary phase diffraction grating.

It is also possible to produce more than three diagnostic light beams2620, 2625, 2630 with the splitting optic 2615. By producing so manydiagnostic light beams, it is possible to record or detect the positionof the current target 110 through the extended target region 115, thusallowing for a more accurate determination of the velocity andacceleration of the current target 110 and also providing a tool forunderstanding the dynamics of the current target 110 as a result of theplasma pushback forces 125.

While a diffractive optic is described above, it is alternativelypossible to use other kinds of optics as the splitting optic 2615. Forexample, the splitting optic 2615 can alternatively or additionallyinclude any one or more of a birefringent crystal, an intensity beamsplitter, a polarization beam splitter, or a dichroic beam splitter.

The diagnostic system 2605 also includes a detection module 2635 thatreceives the light 2640, 2645, 2650 reflected from the current target110 as it passes across the respective diagnostic light beams 2620,2625, 2630. The detection module 2635 includes an imaging lens 2637 anda two-dimensional recording device 2636. The imaging lens 2637 capturesas much of the light 2640, 2645, 2650 as possible and focuses it to animage plane of the image recording device 2636.

The image recording device 2636 is a camera that captures atwo-dimensional representation (image) of each reflected light 2640,2645, 2650. The camera 2636 outputs the set of two-dimensional images tothe control system 2270, which uses the set of two-dimensional images todetermine moving properties of the target 110 along not only the Xdirection but also the Y and Z directions, as discussed above. Thecamera 2636 should be a “high speed” camera in that it is fast enough todetect, record, and output the two-dimensional image of each reflectedlight 2640, 2645, 2650 for a particular target 110 prior to the nexttarget entering the extended target region 115.

The camera 2636 captures each image of the reflected light 2640, 2645,2650 and outputs the set of images to the control system 2270, whichanalyzes the image set and calculates a centroid of each ROI within theimages so as to determine a position of the target 110 along each of theX, Y, and Z directions of the chamber 175, as discussed above.

Referring to FIG. 27, a diagnostic system 2705 is designed to detectboth a one-dimensional characteristic or value (such as the number ofphotons, as discussed above) and two-dimensional representations of thelight produced from the interaction between the diagnostic probe and thetarget 110. To this end, the diagnostic system 2705 is designedsimilarly to the diagnostic system 2605 except that its detection module2735 is designed to split the light 2740, 2745, 2750 so that a portionof this light impinges upon a photodiode 2738 and a portion of the lightimpinges upon the two-dimensional recording device 2736. In this way,the photodiode 2738 captures a one-dimensional aspect of each reflectedlight 2740, 2745, 2750; for example, the photodiode 2738 captures thenumber of photons and outputs a voltage signal that corresponds to thenumber of photons. The photodiode 2738 operates similarly to thephotodiode of the detection module 535 described above. Thetwo-dimensional recording device 2736 operates similarly to therecording device 2636 described above.

The control system 170 connected to the diagnostic system 2705 includesa detection sub-controller 600 or 800 such as shown in FIGS. 6 and 8 forreceiving and processing the pulses from the photodiode 2738 todetermine the time stamps and therefore the time differences for use incalculating the moving properties. The control system 170 connected tothe diagnostic system 2705 also includes a diagnostic sub-controller2200 such as shown in FIG. 22 for receiving and processing the imagesoutput from the two-dimensional recording device 2736. The controlsystem 170 can determine the position P of the target 110 along the Xdirection of the chamber 175 at various locations along the trajectorybased on the data obtained from the photodiode 2738. The control system170 can determine the position P of the target 110 along the Y and Zdirections of the chamber 175 based on the data obtained from thetwo-dimensional recording device 2736 in a manner similar to thatdiscussed above with respect to FIGS. 21A and 21B.

Referring to FIG. 28, a procedure 2800 is performed by the EUV lightsource 100 (under control of the control system 2270) for compensatingfor plasma pushback forces 125 on the target 110. The procedure 2800includes releasing a plurality of targets 110 along their respective atrajectory trajectories toward the target space 120 (2805). The targetspace 120 is positioned to receive the plurality of radiation pulses135. Prior to the target 110 reaching the target space 120 and after aprior and adjacent target 110P has interacted with a prior radiationpulse 135P in the target space 120, a plurality of diagnostic lightprobes (such as probes 2120, 2130) are interacted with the target 110 atdiagnostic locations (such as locations L_(TR1), L_(TR2)) along thetrajectory TR of the target 110 (2810). Under control of the controlsystem 2270, the detection module 2135 detects a plurality oftwo-dimensional representations (such as two-dimensional images) oflight (such as light 2140, 2150) produced due to the interactionsbetween the target 110 and the diagnostic light probes (2815). Thecontrol system 2270 analyzes the detected two-dimensionalrepresentations (for example, the images) (2820). The control system2270 (via the decision module 2274) determines one or more movingproperties of the target 110 based on the analysis of the detectedtwo-dimensional representations (2825).

The plurality of diagnostic light probes that interact with the target110 at the diagnostic locations (2810) can be any of the diagnosticlight probes described herein, such as, for example, the diagnosticlight probes 2420, 2430 of FIG. 24, the diagnostic light probe 2520 ofFIG. 25, or the diagnostic light probes 2620, 2625, 2630 of FIG. 26.

The control system 2270 analyzes the two-dimensional images as follows.For example, the input sub-module 2300) can convert the data of theimages into a format suitable for processing. The pre-processingsub-module 2305 prepares the images by, for example, removing backgroundnoise, filtering the images, and amplifying the signals. The imagesub-module 2310 determines one or more regions of interest (ROIs) withineach image, where each ROI corresponds to a location of the target 110along its trajectory TR. The image sub-module 2310 also calculates anencircled energy of each ROI in the image and calculates a centroid ofeach region of interest. The output sub-module 2315 prepares thecalculated data (such as the area and centroid of the ROIs) for outputto the decision module 2274.

The decision module 2274 also determines whether any of the movingproperties are outside an acceptable range (2830) as well as determinewhether aspects of the light source 100 need to be adjusted if any ofthe moving properties are outside of an acceptable range (2835). Forexample, the decision module 2274 may determine that a timing of theradiation pulse 135 or a direction at which the radiation pulse 135travels needs to be adjusted so that the radiation pulse 135 and thepresent target 110′ efficiently interact with each other (2840). Thedecision module 2274 may determine that an aspect of the target deliverysystem 145 needs to be adjusted to compensate for some long term dynamicissue with the targets 110. After adjustments are made (such as at2840), the radiation pulse 135 is directed toward the present target110′ while the present target 110′ is in the target space 120 to therebyinteract the radiation pulse 135 with the present target 110′ (2845).

The one or more moving properties of the target can be determined forany of the system coordinates; for example, the X, Y, or Z directions ofthe chamber 175. The moving properties of the target 110 that aredetermined include one or more of a position, a velocity, and anacceleration of the target along any of the X, Y, or Z directions of thechamber 175, as discussed above.

The detected two-dimensional image can be analyzed by identifying one ormore regions of interest within the image, such regions of interestcorresponding to a location of the target 110 within the image, andcalculating a central region or moment (centroid) for each identifiedregion of interest.

The two-dimensional representations of light can be detected andanalyzed and the one or more moving properties of the target can bedetermined for a specific target prior to that specific target enteringthe target space 120. Moreover, in some implementations, the target 110interacts with the plurality of diagnostic probes 2140, 2150 while thetarget 110 is being influenced by plasma pushback forces 125.

If a photodiode 2738 is also implemented in the diagnostic system 2705then the procedure 2800 can also include detecting a time associatedwith each interaction between the target and a diagnostic probe;analyzing the detected times; and determining one or more movingproperties of the target along the axial direction (X direction) basedon the analysis of the detected times.

A photodiode 2738 (used in conjunction with the two-dimensionalrecording device 2736) can be used for the purposes of time stamping andtherefore could be used to provide a trigger source 2271 of the controlsystem 2270 for controlling timing aspects of the diagnostic system2705. For example, the time stamps could be used to trigger one or moreof the detection module 2735 and the probe module 2700.

Other implementations are within the scope of the following claims.

In other implementations, the moving property of the current target 110that is detected is a speed of the current target 110, a direction ortrajectory of the current target 110, and an acceleration of the currenttarget 110.

In some implementations, the diagnostic system 105 is arranged toprovide the one or more diagnostic probes 107 so that they interact withthe target 110 in the first region 165 or partially within the firstregion 165. For example, the diagnostic system 2105 can be arranged inthis manner.

What is claimed is:
 1. A diagnostic apparatus comprising: a probe moduleconfigured to direct probe light toward a target trajectory of a targetthat is traveling toward a target space, the probe light having anextent that defines a probe distance D_(P) along the target trajectoryand the probe light interacting with a current target prior to thecurrent target entering the target space; and a detection modulearranged relative to the probe module, the detection module including aplurality of detection apparatuses each configured to detect a pluralityof two-dimensional representations of the current target, eachtwo-dimensional representation of the current target being produced fromthe interaction between the probe light and the current target.
 2. Thediagnostic apparatus of claim 1, wherein the two-dimensionalrepresentations of the current target detected at a detection apparatusare produced from an interaction between probe light and the currenttarget at a plurality of locations and times.
 3. The diagnosticapparatus of claim 1, further comprising a control system incommunication with the detection module and configured to analyze anoutput from the detection module and determine one or more properties ofthe current target based on the analysis.
 4. The diagnostic apparatus ofclaim 3, wherein the control system is configured to send a signal to alight source instructing the light source to adjust one or morecharacteristics of a radiation pulse directed to the target space basedon the output from the detection module.
 5. The diagnostic apparatus ofclaim 1, wherein each detection apparatus comprises a camera, ahigh-speed camera, a charged coupled device, an infrared camera, acomplementary metal-oxide semiconductor, or a photodiode.
 6. Thediagnostic apparatus of claim 1, wherein each two-dimensionalrepresentation of the current target detected at a detection apparatusincludes a shadow of the current target obscuring at least a portion ofthe probe light.
 7. The diagnostic apparatus of claim 1, wherein eachtwo-dimensional representation of the current target detected at adetection apparatus includes light from the probe light scattering off asurface of the current target or a reflection of an image of the currenttarget.
 8. The diagnostic apparatus of claim 1, wherein the probe moduleis configured to direct the probe light toward the target trajectoryprior to the current target interacting with a radiation pulse in thetarget space.
 9. The diagnostic apparatus of claim 1, further comprisinga control system in communication with the detection module andconfigured to reconstruct the trajectory of the target in all three ofthe X, Y, Z directions based on the output from the detection module.10. The diagnostic apparatus of claim 1, wherein the probe lightincludes one or more probe light beams, each probe light beam being acollimated beam when it interacts with the target.
 11. The diagnosticapparatus of claim 1, wherein the probe light includes one or more probelight beams, each probe light beam being a focused beam such that thebeam waist of the probe light beam overlaps the target trajectory. 12.The diagnostic apparatus of claim 1, wherein each detection apparatushas a frame rate that is greater than or equal to a rate at whichtargets are generated and directed toward the target space.
 13. Thediagnostic apparatus of claim 1, wherein each detection apparatusincludes a two-dimensional imaging array that lies in a plane, a firstdetection apparatus having a first two-dimensional imaging array lyingin a plane defined by an axis that is the Z direction and an axis thatis an axis lying in the XY plane; and a second detection apparatushaving a second two-dimensional imaging array lying in a plane definedby an axis that is the Z direction and another axis lying in the XYplane.
 14. A diagnostic method comprising: prior to a current targetreaching a target space, interacting diagnostic light with the currenttarget at a plurality of locations along the target trajectory; anddetecting, at each of a plurality of distinct image planes, a pluralityof two-dimensional representations of the current target produced due tothe interaction between the current target and the diagnostic light atthe plurality of locations along the target trajectory.
 15. Thediagnostic method of claim 14, wherein interacting the diagnostic lightwith the current target at the plurality of locations along the targettrajectory comprises interacting the diagnostic light with the currenttarget after a prior and adjacent target has entered the target space.16. The diagnostic method of claim 14, further comprising determiningone or more of a position, a speed, a velocity, and an acceleration ofthe current target based on the detection of the plurality oftwo-dimensional representations at each of the plurality of distinctimage planes.
 17. The diagnostic method of claim 14, further comprisingdetermining one or more properties of the current target based on thedetection of the plurality of two-dimensional representations at each ofthe plurality of distinct image planes, and sending a signal to a lightsource instructing the light source to adjust one or morecharacteristics of a radiation pulse directed to the target space basedon the determination of the one or more properties.
 18. The diagnosticmethod of claim 17, wherein interacting the diagnostic light with thecurrent target occurs prior to an interaction between the radiationpulse and the current target in the target space.
 19. A diagnosticapparatus comprising: a probe module configured to direct probe lighttoward a target trajectory of a target that is traveling toward a targetspace, the probe light having an extent that defines a probe distanceD_(P) along the target trajectory and the probe light interacting with acurrent target prior to the current target entering the target space; adetection module arranged relative to the probe module, the detectionmodule including a plurality of detection apparatuses each configured todetect a plurality of two-dimensional representations of the currenttarget, each two-dimensional representation of the current target beingproduced from the interaction between the probe light and the currenttarget; and a control system in communication with the probe module andthe detection module, the control system comprising an analysis moduleconfigured to: receive the detected two-dimensional representations fromthe detection module; identify, in each two-dimensional representation,one or more regions of interest, where each region of interestcorresponds to a location of the current target; and calculate an areaand a centroid of each region of interest.
 20. The diagnostic apparatusof claim 19, wherein the control system comprises a trigger sourceconfigured to provide one or more trigger signals to the detectionmodule, the trigger signals instructing the detection apparatuses torecord the respective two-dimensional representations.